Automotive Fuels WERNER DABELSTEIN, Shel Shelll Glo Global bal Sol Soluti utions ons (Deu (Deutsc tschla hland) nd) Gmb GmbH H (ret (retire ired), d), Ham Hambur burg, g, Germany ARNO REGLITZKY, Shell Global Solutions (Deutschland) GmbH (retired), Hamburg, Germany ¨ TZE, Shell Global Solutions Solutions (Deutsch (Deutschland) land) GmbH, Hambur Hamburg, g, German Germany y ANDREA S CHU
KLAUS REDERS, Shell Shell Global Solutions (Deutschland) (Deutschland) GmbH (retir (retired), ed), Hambur Hamburg, g, Germany
1. 1.1. 1. 1. 1.2. 1.2. 2. 2.1. 2.2. 3. 3.1. 3. 1. 3.1. 3. 1.1. 1. 3.1. 3. 1.2. 2. 3.1.3. 3.1 .3. 3.1.4. 3.1. 4. 3.1. 3. 1.5. 5. 3.2. 3. 2. 3.2. 3. 2.1. 1. 3.2. 3. 2.2 2. 3.2. 3. 2.3. 3. 3.2. 3. 2.4. 4. 3.2. 3. 2.5. 5. 3.2. 3. 2.6. 6. 3.2. 3. 2.7. 7. 3.2.8 3.2 .8.. 3.2.9 3.2 .9.. 3.2. 3. 2.10 10.. 4. 4.1. 4. 1. 4.1. 4. 1.1. 1. 4.1. 4. 1.2. 2. 4.1.3 4.1 .3.. 4.1.4 4.1 .4.. 4.1. 4. 1.5. 5. 4.1. 4. 1.6. 6. 4.1. 4. 1.7. 7. 4.1. 4. 1.8. 8.
History . . . . . . . . . . . . . . . . . . . . . . . . . . . Thee Sp Th Spar ark k Ig Igni niti tion on (O (Ott tto) o) En Engi gine ne an and d It Itss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel . Fuel Thee Di Th Dies esel el En Engi gine ne an and d It Itss Fu Fuel el . . . . . . . . Engine Technology . . . . . . . . . . . . . . . . . . Otto Engines . . . . . . . . . . . . . . . . . . . . . . Diesel Engines . . . . . . . . . . . . . . . . . . . . . Fuel Fu el Co Comp mpos osit itio ion n an and d En Engi gine ne Ef Effic ficie ien ncy . Qual Qu alit ity y As Aspe pect ctss of Ga Gaso soli line ne . . . . . . . . . . . Octa Oc tane ne Qu Qual alit ity y . . .. .. . .. .. .. .. .. .. .. Vola Vo lati tili lity ty.. . . . . . . . . . . . . . . . . . . . . . . . . . Fuell Com Fue Compo posit sition ion to Red Reduce uce Tox Toxici icity ty an and d Exhaus Exh austt Emissi Emission ons. s. . . . . . . . . . . . . . . . . . . Stabi Sta bili lity ty,, Cle Clean anli line ness ss,, etc. etc. . . . . . . . . . . . . . Perf Pe rfor orma manc ncee Add Addit itiv ives. es. . . . . . . . . . . . . . . . Qual Qu alit ity y As Aspe pect ctss of Di Dies esel el Fu Fuel elss . . . . . . . . Igni Ig niti tion on Qu Qual alit ity. y. . . . . . . . . . . . . . . . . . . . . Dens De nsit ity y. .. . .. .. . . . . . . . . . . . . . . . . . . . Sulf Su lfur ur Co Cont nten ent. t. . . . . . . . . . . . . . . . . . . . . . Cold Co ld Fl Flow ow Pr Prop oper erti ties. es. . . . . . . . . . . . . . . . . Lubr Lu bric icit ity. y. . . . . . . . . . . . . . . . . . . . . . . . . . Visc Vi scos osit ity. y. . . . . . . . . . . . . . . . . . . . . . . . . . Vola Vo lati tili lity ty.. . . . . . . . . . . . . . . . . . . . . . . . . . Diesel Die sel Fue Fuell Sta Stabil bility ity,, Cle Clean anlin liness ess,, etc. . . . . Diesel Die sel Fue Fuell Eff Effect ectss on Exh Exhau aust st Emi Emissi ssion ons. s. . Perf Pe rfor orma manc ncee Addi Additi tive ves. s. . . . . . . . . . . . . . . . Fuel Components . . . . . . . . . . . . . . . . . . . Gaso Ga soli line ne Co Comp mpon onen ents ts . . . . . . . . . . . . . . . Strai Str aigh ghtt-Ru Run n Gas Gasol olin inee . . . . . . . . . . . . . . . . Ther Th erma mall lly y Cra Crack cked ed Ga Gaso soli line ne . . . . . . . . . . . Cataly Cat alytic ticall ally y Cra Crack cked ed Gas Gasoli oline. ne. . . . . . . . . . Cataly Cat alytic tic Ref Reform ormate ate (Pl (Platf atform ormat ate) e) . . . . . . . Isom Is omer erat atee . . . . . . . . . . . . . . . . . . . . . . . . . Alky Al kyla late te . . . . . . . . . . . . . . . . . . . . . . . . . . Poly Po lyme merr Gas Gasol olin inee . . . . . . . . . . . . . . . . . . . Oxyg Ox ygen enat ates es . . . . . . . . . . . . . . . . . . . . . . . .
426 42 6 426 426 426 42 6 427 42 7 427 42 7 429 42 9 430 430 43 0 430 43 0 431 43 1 432 432 433 43 3 433 43 3 433 43 3 434 43 4 434 434 43 4 434 43 4 435 43 5 435 43 5 435 43 5 435 43 5 435 43 5 436 43 6 436 43 6 437 43 7 437 43 7 438 43 8 438 43 8 438 43 8 438 43 8 439 43 9 439 43 9 439 43 9
2012 20 12 Wil Wiley ey-V -VCH CH Ve Verl rlag ag Gm GmbH bH & Co Co.. KG KGaA aA,, We Wein inhe heim im
DOI: 10.1002/14356007.a16_719. 10.1002/14356007.a16_719.pub2 pub2
4.2. 4.2. 4.2. 4. 2.1. 1. 4.2. 4. 2.2. 2. 4.2. 4. 2.3. 3. 4.2. 4. 2.4. 4. 4.2. 4. 2.5. 5. 4.2. 4. 2.6. 6. 5. 5.1. 5. 1. 5.1. 5. 1.1. 1. 5.1. 5. 1.2. 2. 5.1. 5. 1.3. 3. 5.1. 5. 1.4. 4. 5.1. 5. 1.5. 5. 5.1.6. 5.1 .6. 5.1.7. 5.1.7. 5.1. 5. 1.8. 8. 5.1.9. 5.1 .9. 5.2. 5. 2. 5.2.1. 5.2 .1. 5.2. 5. 2.2. 2. 5.2. 5. 2.3. 3. 5.2. 5. 2.4. 4. 5.2. 5. 2.5. 5. 5.2.6. 5.2 .6. 5.2.7. 5.2. 7. 5.2. 5. 2.8. 8. 5.2. 5. 2.9. 9. 5.2. 5. 2.10 10.. 6. 7. 8.
Diesel Dies el Fu Fuel el Co Comp mpon onen ents ts.. . . . . . . . . . . . . . Stra St raig ight ht-R -Run un Mi Midd ddle le Di Dist stil illa late te . . . . . . . . . . Ther Th erma mall lly y Cra Crack cked ed Ga Gass Oil Oil . . . . . . . . . . . . Cata Ca taly lyti tica call lly y Cra Crack cked ed Ga Gass Oil Oil . . . . . . . . . . Hydr Hy droc ocra rack cked ed Ga Gass Oil Oil . . . . . . . . . . . . . . . . Kero Ke rose sene. ne. . . . . . . . . . . . . . . . . . . . . . . . . . Synt Sy nthe heti ticc Die Diese sell Fuel. Fuel. . . . . . . . . . . . . . . . . Fuel Additives . . . . . . . . . . . . . . . . . . . . . Gaso soli lin ne Ad Add dit itiv ives es . . . . . . . . . . . . . . . . . . Corr Co rros osio ion n Inh Inhib ibit itor ors. s. . . . . . . . . . . . . . . . . . Dete De terg rgen ents. ts. . . . . . . . . . . . . . . . . . . . . . . . . Anti An tiox oxid idan ants ts . . . . . . . . . . . . . . . . . . . . . . . Meta Me tall Dea Deact ctiv ivat ator orss . . . . . . . . . . . . . . . . . . Anti An ti-I -Ici cing ng Ad Addi diti tive vess . . . . . . . . . . . . . . . . . Addit Ad ditive ivess for Com Combat bating ing Com Combu busti stion on Chamb Cha mber er Depo Deposit sitss . . . . . . . . . . . . . . . . . . . Valve Va lve Sea Seatt Rec Recess ession ion Pro Protec tectio tion n Add Additi itives ves . Anti An tikn knoc ock k Age Agent ntss . . . . . . . . . . . . . . . . . . . Dehaz De hazers ers and Ant Antist istati aticc Add Additi itives ves . . . . . . . . . . . . . . . . . . . . Addi Ad diti tive vess fo forr Di Dies esel el Fu Fuel el . Igniti Ign ition on Imp Improv rovers ers (Ce (Cetan tanee Impr Improv overs ers)) . . . . Dete De terg rgen entt Add Addit itiv ives. es. . . . . . . . . . . . . . . . . . Cold Co ld Fl Flow ow Ad Addi diti tive vess . . . . . . . . . . . . . . . . . Lubr Lu bric icit ity y Add Addit itiv ives es . . . . . . . . . . . . . . . . . . Anti An tifo foam am Ad Addi diti tive vess . . . . . . . . . . . . . . . . . . Addit Ad ditive ivess for Inc Increa reasin sing g Sto Storag ragee Sta Stabil bility– ity– Anti An tiox oxid idan ants ts . . . . . . . . . . . . . . . . . . . . . . . Deha De haze zers. rs. . . . . . . . . . . . . . . . . . . . . . . . . . Bioc Bi ocid ides es . . . . . . . . . . . . . . . . . . . . . . . . . . Anti An tist stat atic ic Ad Addi diti tive vess . . . . . . . . . . . . . . . . . . Reod Re odor oran ants ts . . . . . . . . . . . . . . . . . . . . . . . . Fuel Fu el St Stan anda dard rdiz izat atio ion n an and d Te Test stin ing g . . . . . . Stor St ora age an and d Tr Tra ansp spor orttati tio on . . . . . . . . . . . Alternative Fuels . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .
439 439 439 43 9 440 44 0 440 44 0 441 44 1 441 44 1 441 44 1 441 44 1 442 44 2 442 44 2 443 44 3 444 44 4 445 44 5 445 44 5 446 446 446 44 6 446 44 6 446 44 6 446 44 6 446 44 6 447 44 7 447 44 7 449 44 9 450 45 0 450 450 450 45 0 451 45 1 451 45 1 451 45 1 451 45 1 452 45 2 453 45 3 457 45 7
426
Automotive Fuels
1. His Histor tory y 1.1. The Spark Ignition Ignition (Otto) (Otto) Engine Engine and Its Fuel In 1876 NIKOLAUS AUGUST OTTO develope developed d a stationary, statio nary, single single-cylin -cylinder, der, fourfour-stroke stroke engin engine, e, that ran on coal gas. This invention, which was late la terr na named med af afte terr hi him, m, se sett a de deve velo lopme pment nt in motion that substantially shaped the industrial age and has not yet come to a halt [1, 2]. About ten years later the four-stroke engine was first used to drive vehicles (Daimler and Maybach automobiles, Benz patent car). The first vehicle engines ran on so-called light gasoli gasoline ne which 3 had a low density ( < 700 kg/m ) and a very low final boiling point (ca. 85 C). Light gasoline was available as a cleaning agent at pharmacies and an d co coul uld d be ev evap apor orat ated ed in th thee fir first st su surf rfac acee carburetors carbu retors.. The development development of spray nozzle carburetors finally enabled the use of straightrun gasoline with a broader boiling range and higher final boiling point [3]. A higher proportion of crude petroleum could thus be used as a fuel, and the rising demand for gasoline could be satisfied. After the development of a reliable mixture prepar pre paratio ation n sys system tem,, att attent ention ion foc focuse used d on the furthe fur therr imp improv roveme ement nt of the eng engine ine pow power er by incr in crea easin sing g th thee co comp mpre ressi ssion on ra ratio tio (f (fro rom m ca ca.. 1918 on). The increase in compression ratio was show sh own n to be lim limite ited d by ab abno norm rmal al co combu mbust stio ion, n, so so-calle ca lled d en engi gine ne kn knoc ock, k, wh whic ich h ca can n da damag magee th thee engi en gine ne.. Si Sinc ncee it wa wass ob obse serv rved ed th that at kn knoc ockin king g combustion can be related to gasoline formulation, systematic research began to improve the knock resistance of fuels. Discovery of the strong antiknock effect of tetraethyllead by MIDGLEY and BOYD in 1922 was a milestone in the development of gasolines. A little later, refine refinery ry conve conversion rsion processes were established that allowed the demand for gasoline to be met by improving the yield, and which also led le d to a fu furt rthe herr im impr prov ovem emen entt in an anti tikn knoc ock k perfor per forman mance, ce, soso-cal called led oct octane ane qua quality lity (! Oil Refining). The further advances in the development of engine technology and in the quality of gasoline were mutually dependent. From 1950 on, increasing attention was given to the further impr im prov ovem emen entt of ga gaso solin linee by th thee ad addi ditio tion n of smal sm alll qu quan anti titi ties es of hi high ghly ly ac acti tive ve ad addi diti tive vess (see Section 5.1).
Vol. 4
Since 1970, measures for improving the environm vir onment ental al com compat patibi ibility lity of aut automo omotiv tivee use have become increasingly important worldwide. The first step was the successive reduction in the lead le ad ad addi diti tive ve co cont nten entt of ga gaso soli line nes. s. La Late ter, r, completely unleaded gasolines were developed, which were a prerequisite for the introduction of catalytic converters. Thee re Th redu duct ctio ion n of su sulf lfur ur co cont nten entt in ga gaso solin linee ha hass occurred more recently and it is still ongoing. This measure improves the efficiency and durability of modern exhaust gas aftertreatment devices and reduces the emission of sulfur compounds. In this context ‘‘sulfur free’’ gasolines (less than 10 ppm sulfur) have been made available ab le on a wid wider er sc scal ale. e. In co coun untr tries ies wi with th ta tax x incentives for ‘‘sulfur free’’ fuels like Germany this fuel quality has a high market share. The limitation of benzene, other aromatics, and olefinss co fin conc ncen entr trat atio ion n in ga gaso solin linee is in inte tend nded ed to reduce the resulting-post catalyst exhaust emissions. The worldwide rising demand for oil products has led to noticeable price increases and because becau se of this development development alternative alternative fuels or fuel components (e.g., alcohols from biomass) have again achieved a certain significance.
1.2. The Diesel Diesel Engine Engine and Its Fuel The diesel engine, patented in 1892 by R UDOLF DIESEL, is characterized by self-ignition of the fuell [4] fue [4].. Sel Self-i f-igni gnitio tion n is ach achiev ieved ed by com compre pressin ssing g thee in th inco comi ming ng ai airr to su such ch an ex exte tent nt th that at th thee result res ulting ing tem temper peratu ature re inc increa rease se is suf suffici ficient ent to ignite the fuel. In th thee de deve velo lopm pmen entt of hi hiss en engi gine ne,, DIESEL’S aim was an engine operation as close as possible to the Carnot Carnot cycle – which gives gives the highest highest theoretica ret icall effi efficie ciency ncy – wit with h the result result tha thatt the first diesel engine built in 1893 at the Maschinenfabrik ri k Au Augs gsbu burg rg N€ urnberg urnber g AG (MAN (MAN)) oper operate ated d at a hither hit herto to una unachi chieve eved d effi efficie ciency ncy of 26. 26.2%. 2%. The powe po werr of th this is en engi gine ne wa wass 17 17.8 .8 me metr tric ic ho hors rsep epow ower er (13.1 kW ) at 194 194 rpm, and and it ran on a mixture mixture of of gasoline and lamp oil. After 1897, experiments with other fuels started, which had little success at first. Thee in Th inje ject ctio ion n of sm smal alll qu quan antit titie iess of fu fuel el at hi high gh pres pr essu sure re ca caus used ed th thee gr great eates estt pr prob oblem lem in th thee in initi itial al phase of the development. Blowing the fuel in with compressed air, which was tried at first, did
426
Automotive Fuels
1. His Histor tory y 1.1. The Spark Ignition Ignition (Otto) (Otto) Engine Engine and Its Fuel In 1876 NIKOLAUS AUGUST OTTO develope developed d a stationary, statio nary, single single-cylin -cylinder, der, fourfour-stroke stroke engin engine, e, that ran on coal gas. This invention, which was late la terr na named med af afte terr hi him, m, se sett a de deve velo lopme pment nt in motion that substantially shaped the industrial age and has not yet come to a halt [1, 2]. About ten years later the four-stroke engine was first used to drive vehicles (Daimler and Maybach automobiles, Benz patent car). The first vehicle engines ran on so-called light gasoli gasoline ne which 3 had a low density ( < 700 kg/m ) and a very low final boiling point (ca. 85 C). Light gasoline was available as a cleaning agent at pharmacies and an d co coul uld d be ev evap apor orat ated ed in th thee fir first st su surf rfac acee carburetors carbu retors.. The development development of spray nozzle carburetors finally enabled the use of straightrun gasoline with a broader boiling range and higher final boiling point [3]. A higher proportion of crude petroleum could thus be used as a fuel, and the rising demand for gasoline could be satisfied. After the development of a reliable mixture prepar pre paratio ation n sys system tem,, att attent ention ion foc focuse used d on the furthe fur therr imp improv roveme ement nt of the eng engine ine pow power er by incr in crea easin sing g th thee co comp mpre ressi ssion on ra ratio tio (f (fro rom m ca ca.. 1918 on). The increase in compression ratio was show sh own n to be lim limite ited d by ab abno norm rmal al co combu mbust stio ion, n, so so-calle ca lled d en engi gine ne kn knoc ock, k, wh whic ich h ca can n da damag magee th thee engi en gine ne.. Si Sinc ncee it wa wass ob obse serv rved ed th that at kn knoc ockin king g combustion can be related to gasoline formulation, systematic research began to improve the knock resistance of fuels. Discovery of the strong antiknock effect of tetraethyllead by MIDGLEY and BOYD in 1922 was a milestone in the development of gasolines. A little later, refine refinery ry conve conversion rsion processes were established that allowed the demand for gasoline to be met by improving the yield, and which also led le d to a fu furt rthe herr im impr prov ovem emen entt in an anti tikn knoc ock k perfor per forman mance, ce, soso-cal called led oct octane ane qua quality lity (! Oil Refining). The further advances in the development of engine technology and in the quality of gasoline were mutually dependent. From 1950 on, increasing attention was given to the further impr im prov ovem emen entt of ga gaso solin linee by th thee ad addi ditio tion n of smal sm alll qu quan anti titi ties es of hi high ghly ly ac acti tive ve ad addi diti tive vess (see Section 5.1).
Vol. 4
Since 1970, measures for improving the environm vir onment ental al com compat patibi ibility lity of aut automo omotiv tivee use have become increasingly important worldwide. The first step was the successive reduction in the lead le ad ad addi diti tive ve co cont nten entt of ga gaso soli line nes. s. La Late ter, r, completely unleaded gasolines were developed, which were a prerequisite for the introduction of catalytic converters. Thee re Th redu duct ctio ion n of su sulf lfur ur co cont nten entt in ga gaso solin linee ha hass occurred more recently and it is still ongoing. This measure improves the efficiency and durability of modern exhaust gas aftertreatment devices and reduces the emission of sulfur compounds. In this context ‘‘sulfur free’’ gasolines (less than 10 ppm sulfur) have been made available ab le on a wid wider er sc scal ale. e. In co coun untr tries ies wi with th ta tax x incentives for ‘‘sulfur free’’ fuels like Germany this fuel quality has a high market share. The limitation of benzene, other aromatics, and olefinss co fin conc ncen entr trat atio ion n in ga gaso solin linee is in inte tend nded ed to reduce the resulting-post catalyst exhaust emissions. The worldwide rising demand for oil products has led to noticeable price increases and because becau se of this development development alternative alternative fuels or fuel components (e.g., alcohols from biomass) have again achieved a certain significance.
1.2. The Diesel Diesel Engine Engine and Its Fuel The diesel engine, patented in 1892 by R UDOLF DIESEL, is characterized by self-ignition of the fuell [4] fue [4].. Sel Self-i f-igni gnitio tion n is ach achiev ieved ed by com compre pressin ssing g thee in th inco comi ming ng ai airr to su such ch an ex exte tent nt th that at th thee result res ulting ing tem temper peratu ature re inc increa rease se is suf suffici ficient ent to ignite the fuel. In th thee de deve velo lopm pmen entt of hi hiss en engi gine ne,, DIESEL’S aim was an engine operation as close as possible to the Carnot Carnot cycle – which gives gives the highest highest theoretica ret icall effi efficie ciency ncy – wit with h the result result tha thatt the first diesel engine built in 1893 at the Maschinenfabrik ri k Au Augs gsbu burg rg N€ urnberg urnber g AG (MAN (MAN)) oper operate ated d at a hither hit herto to una unachi chieve eved d effi efficie ciency ncy of 26. 26.2%. 2%. The powe po werr of th this is en engi gine ne wa wass 17 17.8 .8 me metr tric ic ho hors rsep epow ower er (13.1 kW ) at 194 194 rpm, and and it ran on a mixture mixture of of gasoline and lamp oil. After 1897, experiments with other fuels started, which had little success at first. Thee in Th inje ject ctio ion n of sm smal alll qu quan antit titie iess of fu fuel el at hi high gh pres pr essu sure re ca caus used ed th thee gr great eates estt pr prob oblem lem in th thee in initi itial al phase of the development. Blowing the fuel in with compressed air, which was tried at first, did
Vol. 4
Automotive Fuels
not solve the problem satisfactorily. High-pressure liquid injection was developed in 1910; and in 1927, large-scale large-scale production of fuel injection syst sy stem emss st star arte ted d an and d th thee wa way y wa wass op open ened ed fo forr br broa oad d expansion of the diesel engine. MAN and Daimler – Benz introduced the diesel engine to commercia mer ciall veh vehicl icles es in 192 1923 3 – 192 1924. 4. About About 12 year yearss later, Daimler – Benz introduced the first passenger car diesel engine (type 260 D, 2.6 L, 45 metric horsepower at 3000 rpm). Even the first diesel engines were often run on the so-called gas-oil fraction of crude petroleum (boiling range 200 – 350 C). This type of fuel, which is provided in sufficient quantity main ma inly ly by cr crud udee oi oill di dist still illati ation on at mo mode dera rate te prod pr oduc uctio tion n co cost st is ba basi sica cally lly st still ill us used ed to toda day, y, beca be caus usee of it itss su suita itabl blee ig igni nitio tion n qu qual ality ity (s (see ee Section 3.2.1). In addition to high ignition quality (cetane number; numbe r; see Sectio Section n 3.2.1) 3.2.1),, cold flow properties and low ten tenden dency cy tow toward ard cok coking ing and soo sootin ting g became important at an early stage. Duri Du ring ng Wo Worl rld d Wa Warr II II,, la larg rgee qu quan antit tities ies of dies di esel el fu fuel el we were re pr prod oduc uced ed fr from om co coal al in Ge Germa rmany ny.. Mixtures of components of low ignition quality (tar oil) and very high ignition quality (from the Fischer – Tropsch synthesis) were used [3]. Beca Be cause use of th thee in incr crea easin sing g de dema mand nd fo forr lig light ht oi oill produc pro ducts ts tha thatt foll followe owed d Wor World ld War II, gas gas-oi -oill fractions from conversion processes have also been used in diesel fuel. However, these components are generally less suitable because of their lower ignition quality. For environmental protectio tec tion n and imp improv roved ed eng engine ine per perfor forman mance ce a num num-berr of me be meas asur ures es we were re ta take ken n to ra rais isee th thee di diese esell fu fuel el quality. For example, the maximum permitted sulfur sul fur con conten tentt of die diesel sel fue fuell was low lowered eredste stepwi pwise se from 1.0 (1955 in Germany) to 0.035 mass% (2000) and further to 0.005 mass% (2005) in Europe. Europ e. Also a fuel qualit quality y with a sulfur content less le ss th than an 0. 0.00 001 1 ma mass ss% % (s (sul ulfu furr fr free ee)) mu must st be available. The maximum permitted sulfur content in different areas is given below:
United State United Statess EU Japan
0.001 (max averag 0.001 averagee of refiner refinery y producti production) on) mass% mass% 0.005 and 0.001 mass% 0.0015 mass%
The re The redu ducti ction on of su sulf lfur ur in th thee di dies esel el fu fuel el helped hel ped als also o in the intr introdu oducti ction on of par partic ticulat ulatee tra traps ps in the exhaust systems.
427
Synthetic compo Synthetic components nents with ultraultra-high high ignit ignition ion quality available from, e.g., natural gas synthesis (basicly Fischer-Tropsch synthesis with a newly developed catalyst) are used in some markets to formula for mulate te a die diesel sel fue fuell wit with h sup superio eriorr qua quality lity.. Als Also o high-quality high-q uality compo components nents prod produced uced from bioma biomass ss will shortly be available in limited quantities for commercial use. Reliable engine operation and a reduction in exhaust gas emissions is also the primary aim in the tre treatm atment ent of die diesel sel fuels fuels with mul multifu tifunct nction ional al additiv add itives, es, whi which ch has com comee int into o inc increa reased sed use from 1985 onwards. In addition, acceptability of diesel fuels could be improved (foaming, odor; see Section 5.2).
2. Engi Engine ne Technol Technology ogy 2.1. Otto Engin Engines es The bas The asic ic pri rinc ncip iple le of th thee sp spaark ig igni niti tio on (Otto) engine–a combustion engine with timed igni ig nitio tion n fr from om an ex exte tern rnal al so sour urce ce – ca can n be re re-aliz al ized ed in a nu numb mber er of va vari rian ants ts wi with th di diff ffere erent nt type ty pess of fu fuel el me mete teri ring ng an and d mi mixt xtur uree pr prep epara ara-tionss [5] tion [5].. The most frequently used type is the fourstroke reciprocating Otto engine in which gas exchange processes during these four strokes are controlled by valves. The sequence of strokes is 1. intake intake of the fuel fuel – air mixtur mixture, e, 2. co comp mpre ress ssio ion n of th thee mi mixt xtur uree an and d ti time med d ignition, 3. combu combustion stion and expansion expansion (working stroke), stroke), and 4. exhau exhaust st of combustion combustion gases. To carry out one working stroke the crankshaft turns twice. The valves are operated by a camshaft connected to the crankshaft. The rotar (frequ equent ently ly cal called led the Wanrotary y engin engine e (fr kel engine after its inventor) is a variant of the four-stroke four-s troke engin engine. e. It diffe differs rs from the recip reciprocatrocating in g pi pist ston on en engi gine ne in it itss ro rota tary ry pi pist ston on an and d in th thee us usee of ports for control of the inlet and exhaust gases [6,, 7] [6 7].. The The ro rotar tary y pi pist ston on en engi gine ne ha hass no nott be been en ve very ry successful up to the present, compared to the recipro rec iprocat cating ing eng engine ine bec becaus ausee of its rela relative tively ly high fuel consumpt ptiion and exhaust gas emissions.
428
Automotive Fuels
In the standard two-stroke Otto engine the gas exchanges are also controlled by ports (inlet, transfer, and exhaust ports) in the cylinder. A working cycle is achieved with each rotation of the crankshaft, and high power output at low swept volume is achieved. However, in the twostroke process, scavenging losses must be accepted that lead to relatively high fuel consumption and hydrocarbon-exhaust emission. The two-stroke engine is used only in small motorcycles, outboard engines and light-weight machines [8]. Technically advanced models controlled by valves can overcome many of the disadvantages normally associated with these engines to a certain degree. However, more stringent exhaust gas requirements will reduce the use of two-stroke engines in the future. The Otto engine runs with throttled intake air (quantity governing) and an almost homogeneous, approximately stoichiometric fuel – air mixture. Traditionally, the composition of the mixture depended on the running condition of the engine: i.e., at full load and directly after cold start the engines operated with an excess of fuel (rich mixture) whereas under part load nearer to the stoichiometric mixture. For the preparation of the fuel-air mixture previously carburetors [9] were used. Modern electronic fuel injection systems, are superior to carburetors with respect to performance, fuel consumption, and accuracy of mixture strength control necessary for the efficient use of three-way catalytic converters [10]. Carburetors were therefore gradually replaced by fuel injection systems. Latest development are the so-called direct injected engines, which offer optimum fuel efficiency [11]. The main advantage of this technology is the avoidance of throttling losses at part load conditions. Whereas conventional Otto engines run at fixed air-fuel ratio and adjust power at part load by reducing the cylinder charge by the throttle valve, direct injected engines run in this respect rather like a diesel with almost unthrottled cylinder charge and varying amounts of fuel injected. To ensure proper ignitability of very lean mixtures at low load a locally gasoline enriched layer of air – fuel mixture (‘‘stratified charge’’) must be created. In the Otto engine the combustion process is started by an electric spark. Ignition occurs shortly before the piston has reached the top
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dead center and is advanced with increasing speed to allow a timed combustion beginning shortly before the top dead center [12]. To achieve the highest efficiency in the Otto engine, combustion induced by the ignition spark must develop in a controlled way under all operating conditions. If the resistance of the gasoline against autoignition, i.e., the octane quality of the fuel does not meet the octane number requirement of the engine, after compression by the piston either before or after ignition by the spark plug, autoignition of the yet unburned portion of the mixture occurs. This autoignition (engine knock) leads to a considerably higher combustion rate with a very sharp increase in pressure and temperature (Fig. 1). An anomalous combustion of this type manifests itself by a knocking or pinking noise. Whereas occasional knock that occurs on acceleration usually does not damage the engine, permanently knocking combustion at high speeds and loads over a longer period increases the risk of engine damage. Modern engine knock sensors allow safe engine operation even if gasolines of varying octane qualities are used, because they adjust the spark timing according to octane quality. However, gasolines with low octane quality require retarded ignition which leads to efficiency losses and, e.g., higher exhaust gas temperatures. Apart from design parameters the octane number requirement of an Otto engine varies with the engine speed and the load. Furthermore increasing deposit formation in the combustion chamber with increasing milage can raise the octane number requirement [13]. In designing an engine on the basis of a particular type of fuel, a safety margin must therefore be provided, to
Figure 1. Combustion in theOtto enginea) Without ignition; b) Normal combustion; c) Knocking combustion; d) Topdead center
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avoid knocking combustion even under unfavorable conditions.
2.2. Diesel Engines The diesel engine differs from the Otto engine in the heterogeneous composition of its mixture and the self-ignition of the fuel. The temperature necessary for self-ignition is produced in the compression stroke, in which compression of air to ca. 3.0 – 5.5 MPa leads to temperatures of 700 – 900 C. Fuel is injected into the heated air of the combustion chamber shortly before the end of the compression stroke, where it self-ignites. In principle the diesel engine can be either of the four- or two-stroke type. In practice, four-stroke engines are found in the vast majority of vehicles, whereas a valve-controlled two-stroke process with precompressed air is used in large ship engines. In the diesel engine, air is generally taken in unthrottled and the fuel – air ratio varies with the quantity of fuel injected (quality governing) thus leading to a very lean fuel – air mixture (l > 1.3, for definition of l , see ! Automobile Exhaust Control). Two types of combustion systems have been developed for the diesel engine that differ in the preparation of the combustible gases [14]. Direct injection is characterized by a one-piece combustion chamber, into which fuel is injected without further preparation. Indirect injection is characterized by a two piece combustion chamber and was a mechanical solution to control the rate of combustion. One of the main problems of the early diesel engines was the control of the combustion rate. Depending on the fuel and engine parameters ignition was often delayed and the subsequent rate of combustion more rapid than desired for best efficiency, low noise and emissions, etc. In those days before the development of modern electronic control units, various mechanical solutions were available, such as divided combustion chambers or specific mechanically controlled injection spray patterns to adjust the rate of combustion. Very common, especially for passenger car diesel engines, were concepts with divided combustion chambers, so called indirect injection engines (IDI engines). These precombustion chamber processes, such as the swirl
Automotive Fuels
429
chamber and prechamber processes, have a two-piece combustion chamber. Fuel is injected into the precombustion chamber, partially homogenized with air, and self-ignited. Partially oxidized combustible gases and evaporated fuel reach the main combustion chamber via connecting channels and burn there after further mixing with air. Indirect injection engines were used predominantly in passenger cars. Their engine noise is less harsh, but their fuel consumption is higher than that of the direct injection diesel engine. The split combustion chamber gives rise to pumping losses which impair the efficiency of precombustion chamber engines. This is also brought about by their higher compression ratio leading to higher friction losses. In contrast to the divided combustion chamber engine concepts with a one-piece combustion chamber, i.e., direct injection engines (DI engines), were originally used mainly for heavyduty transport and machinery. Their undivided combustion chamber allows higher engine efficiency. In modern direct injection engines sequentially electronically controlled high-pressure fuel injection systems allow optimum combustion and pressure rise thus avoiding previous problems, e.g., noise or limited speed range. The so-called common rail technology allowing high injection pressures is now well implemented for passenger car diesel engines. The compression ratio of direct injection engines (ca. 15 – 17 : 1) lies near that optimum for most favorable total efficiency, which occurs at a compression ratio of ca. 15 : 1. As mentioned before the diesel fuel is in jected into the combustion chamber near the end of the compression stroke shortly before the piston has reached the top dead center. The period between the beginning of the injection mode and the start of the combustion is called ignition delay. The length of ignition delay depends on ambient conditions, e.g., temperature, design and construction of the engine and the ignition quality of the diesel fuel. Optimum engine operation, necessary for high efficiency, low noise, and exhaust emissions, requires short ignition delay and therefore high diesel fuel ignition quality, i.e., a high cetane number. Prolonged ignition delay causes an unacceptable steep pressure rise causing, e.g., high noise and nitrogen oxide emissions (see Fig. 2) [15, 16].
430
Automotive Fuels
Figure 2. Combustion in the diesel engine a) Noisy combustion; b) Normal combustion; c) Without combustion; d) Ignition delay 1; e) Ignition delay 2; f ) Top dead center; g) Injection period
3. Fuel Composition and Engine Efficiency Because of the different modes of operation of Otto and diesel engines (see Chap. 2), completely different requirements exist for gasoline and diesel fuels and their composition. The effects of important fuel properties on the performance of engines are discussed below. The standardization of minimum requirements derived from them is described in Chapter 6.
3.1. Quality Aspects of Gasoline Themost importantquality criteria forgasoline are its resistance against autoignition (engine knock), i.e., octane quality, the evaporation properties to provide an ignitable air – fuel vapor in the combustionchamber, environmental acceptability andlow toxicity, and also cleanliness and stability. While all of these aspects are influenced by the main gasoline components and thus by refinery technologies, some canalso be improved by additives,i.e., small proportions of organic substances, added to the gasoline mainly at the end of the production process andprior to delivery to the service stations.
3.1.1. Octane Quality
High octane quality is required to allow knockfree combustion in high compression engines to
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enable optimum engine efficiency. While the engine efficiency can be raised generally by increasing the compression ratio – leading to a higher octane requirement of the engine – the production of matching high octane fuels is connected with a higher energy demand in the refinery. The overall optimum efficiency (taking into account engine and refinery) is obtained with the most widely used premium and super gasolines with research octane numbers of 95 to 100 and engines designed for that octane quality. The octane level of a gasoline depends on that of the individual refinery streams and on that of other, e.g., oxygen containing components such as ethers or alcohols. The octane quality of gasoline is described by the octane number. The octane number of a gasoline is determined by comparative measurements of its octane quality and that of binary mixtures having variable concentrations of n-heptane (low octane quality) and 2,2,4-trimethylpentane (isooctane; high octane quality). By definition the octane number of n-heptane is 0 and that of isooctane is 100. The octane numbers of mixtures are given by their percentage by volume of isooctane; octane numbers > 100 can be determined with lead-containing isooctane or toluene-containing mixtures. Octane number determination (e.g., according to EN 25164 or EN 25163) is carried out in singlecylinder, four-stroke test engines specially developed for the purpose. The CFR (Cooperative Fuel Research) engine is most widely used worldwide. The test engines have a mechanically adjustable compressionratio.Thisratioisraisedduringoctane number measurement of a fuel until a certain standardized level of knocking, shown by a socalled knock meter, is obtained. The gasoline’s octane number is determined by finding the nheptane – isooctane mixture that gives the same knocklevel.Determinationoftheoctanenumberis carried out under two different running conditions: 1. Determination of the research octane number (RON): according to EN 25164, ASTM D 2699, in which comparatively mild conditions are used (600 rpm, no prewarming of the mixture, constant ignition timing). 2. Determination of the motor octane number (MON): according to the EN 25163, ASTM D 2700, in which a higher mechanical and thermal load is used (900 rpm, prewarming of the mixture to 150 C, andvariable ignition timing).
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Because of the more severe test conditions the MON is usually lower than the RON; the difference between both is called sensitivity [17]. The RON describes the knocking performance of a gasoline at low and medium engine speeds (‘‘acceleration knocking’’) whereas the MON defines the knock behavior under high speed and load (‘‘high speed knocking’’). The knock rating of a fuel is generally sufficiently characterized by its RON and MON. Historically it was necessary to use an additional characteristic, the front octane number (FON) describing the RON knock rating of the low boiling fuel components. An insufficient FON leads to knocking during acceleration, particularly in carburetor vehicles. This problem does no longer exist in fuel-injected vehicles. For research and development purposes it is also possible to measure the octane quality in vehicle Otto engines under normal road use. The so-called road octane number gives information on the octane quality in vehicle Otto engines under normal road use. The road octane number is specifically related to the vehicle type and is also affected by driving conditions and weather. Usually the road octane number is given by an expression of the type a RON þ b MON þ c, where a, b, and c are constants that depend on the vehicle. The road octane number of a gasoline lies between the RON and MON, nearer the RON under acceleration conditions and approaching the MON under constant speed.
Automotive Fuels
431
3.1.2. Volatility
Fuel volatility, expressed by distillation behavior (EN ISO 3405, ASTM D 86) and vapor pressure, controls the formation of an ignitable air – fuel mixture and the ability of the engine to start and run under cold and hot conditions. For safe operation the fuel must have a balanced distillation performance [13]. A certain proportion of volatile fuel components are needed for troublefree performance at low temperature. An excess of these components leads, however, to hot fuel handling problems with a warm engine or high ambient temperature (see Fig. 3). To ensure trouble-free engine running, the boiling characteristics are usually matched to climatic conditions and change during the year. An excessively high final boiling point can cause deposit formation and lubricant dilution, and consequently engine wear. Gasoline volatility also controls exhaust gas emissions and evaporative emissions. Vehicle design elements such as the activated carbon canister have of course a larger effect on these emissions. However, in a vehicle of given design the volatility of the fuel can contribute to guarantee low emissions. For example, a sufficiently high vapor pressure and high volatility ensures low exhaust gas emissions at low ambient and engine temperatures and sufficiently low vapor pressure reduces evaporative losses at high ambient and fuel system temperatures.
Figure 3. Significance of gasoline distillation properties for engine performance and emissions [19]
432
Automotive Fuels
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3.1.3. Fuel Composition to Reduce Toxicity and Exhaust Emissions
at the combustion chamber surfaces, or partly oxidized due to the high pressures and temperatures. Apart from carbon monoxide, unburned Some of the gasoline constituents, especially hydrocarbons, and nitrogen oxides, other toxic benzene and previously used lead antiknock substances such as aldehydes, benzene, and 1,3additives, are toxic and other components can butadiene are emitted at very low concentraform toxic materials during combustion. Both tions. All of these are oxidized or reduced by the benzene and alkylleads were formerly used to exhaust gas catalysts by more than 95%. Their improve the octane quality of gasolines. Their residual concentration can be influenced by maximum allowed concentration was legally gasoline composition and hence by the gasoline reduced stepwise. While lead has completely formulation in the refinery. Gasolines forming disappeared from gasoline in most countries also less toxic and environmentally harmful material because of its poisoning effects on exhaust gas have been called ‘‘reformulated gasolines’’. catalysts, benzene content has been restricted in The effects of gasoline properties and compoEurope initially to maximum 5 vol% and since sition on exhaust gas and evaporative emissions the year 2000 to 1 vol%. The major German oil have been studied in a wide range of modern companies produced their top quality Super Plus vehicles, partly in joint motor – oil industry since 1995 already with benzene concentrations projects, (e.g., auto – oil programs) in the US below 1 vol% on a voluntary basis. Benzene has and in Europe. The results of the European not been used as an active blending component program are summarized in Tables 1 and 2 [20]. for a long time. It is extracted from the relevant The reduction of sulfur and benzene has a gasoline components (e.g., reformate) mainly by clear-cut positive effect. The other measures, e. distillation and also its formation in the catalytic g., the reduction of total aromatics, the increase reformer is minimized by extraction of ‘‘benzene of mid-range volatility and the use of methyl tert precursors‘‘ in the reformer feed material. butyl ether have positive effects on some of the To keep the octane quality of gasolines with- emission components but negative on others. out lead and benzene above the standardized This applies also to the reduction of vapor preslevel, contents of other nontoxic materials were sure (reduction of butane), which reduces the increased, namely of isomerates, higher aro- evaporative losses but increases emission of CO2 matics, and to some extent ethers (! Methyl and unburned hydrocarbons under cold start Tert&Butyl Ether) and alcohols. conditions. Ideally the gasoline is combusted completely A further objective is to reduce the formation to water and carbon dioxide, however, in prac- of ozone and smog that occurs as a result of tice other unwanted components are emitted photochemical reactions between hydrocarbons because a small proportion of the gasoline and nitrogen oxides. The ozone forming potential (usually less than 1%) leaves the combustion of hydrocarbons differs depending on their strucchamber unburned, e.g., because of quenching ture and size. Thus, the composition of the un-
Table 1. Summary of effects of gasoline properties on emissions in noncatalyst cars (source: EPEFE program)
Change Add oxygenate 0.0 to 2.7 mass% O2 Reduce aromatics 40 to 25 vol% Reduce benzene 3 to 2 vol% Reduce olefins 10 to 5 vol% Reduce sulfur 300 to 100 ppm Reduce Reid vapor pressure 70 to 60 kPa Increase E 100* 50 to 60% ** Increase E 150 85 to 90%
Lead CO
HC exhaust HC evaporation NO x Benzene Butadiene Aldehydes
0 0 0 0 0 0 0 0
!
!!! !
!
0
0
0–! 0 –0 –0 0
0
0
!
!!
0
!!
!
0 0 0 0 0
0
~
0 0
0
0
!!!
þ 0?
!?
0
0 0 0
0
!!
0
~?
0 0 0
~~ ~
0 0 0
0 0 0 0 0
~?
~?
!!
0 ¼ no effect. 0 ¼ 2 to þ 2%. ! or ~ ¼ 2 – 10% effect. !! or ~~ ¼ 10 – 20% effect. !!! or ~~~ ¼ ? ¼ insufficient information. * E 100 ¼ % evaporated at 100 C. ** E 150 ¼ % evaporated at 150 C.
> 20%.
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Automotive Fuels
433
Table 2. Summary of effects of gasoline properties on emissions in catalyst cars (source: EPEFE program)
Change Add oxygenate 0.0 to 2.7 mass% O2 Reduce aromatics 40 to 25 vol% Reduce benzene 3 to 2 vol% Reduce olefins 10 to 5 vol% Reduce sulfur 300 to 100 ppm Reduce Reid vapor pressure 70 to 60 kPa Increase E 100* 50 to 60% ** Increase E 150 85 to 90%
Lead CO
HC exhaust HC evaporation
0 0 0 0 0 0 0 0
!
!!
0–~ 0 –0 0 0
!
!?
0 0
0
!?
!?
0
0
!!
þ 0?
!?
0
!!
0 0
þ 0
NO x Benzene Butadiene Aldehydes
þ 0
0
0
~?
!!
þ 0
~?
0
!!
0
0
0
!!
0 0
!?
!?
!?
!?
0 0
0 0 0
0 0
0 0
!?
!?
~?
0 ¼ no effect. 0 ¼ 2 to þ 2%. ! or ~ ¼ 2 – 10% effect. !! or ~~ ¼ 10 – 20% effect. !!! or ~~~ ¼ ? ¼ insufficient information. * % evaporated at 100 C. ** % evaporated at 150 C.
burned hydrocarbons can be optimized by the composition of the gasoline to reduce the ozone forming potential of evaporative losses and exhaust emissions. As an example the reduction of the highly reactive olefins in the fuel is a measure to reduce the ozone forming potential, however olefins are desired fast burning gasoline components which are also readily converted in the exhaust gas catalysts. The evaluation of the results of the above mentioned programs have led to the production of specific ‘‘reformulated gasolines’’ in some areas, e.g., California and to the stepwise introduction of more stringent gasoline specifications, e.g., in Europe, as listed below.
Aromatics Benzene Olefins Sulfur *
2000 42 vol% 1.0 vol% 18 vol%* 150 ppm
2005 35 vol% 1.0 vol% 18 vol% 50 ppm and 10 ppm
Max. olefin content regular grade 21 vol%
In Germany some mineral oil companies had voluntarily introduced sulfur free gasoline qualities, e.g., Shell Optimax. Since 2005 at least one sulfur free gasoline must be available at the gasoline stations. Currently a tax incentive for sulfur free gasoline ensures that the majority of gasoline sold in Germany is sulfur free. Low and ultra-low sulfur levels can also be found in other countries, e.g., in Japan and in the US. Sulfur free gasolines (sulfur content less than 10 ppm) are required for new highly efficient exhaust gas aftertreatment systems especially for lean burn direct injection engines. The new specifications require changes
~~
> 20%.
to the refinery units including substantial investment. 3.1.4. Stability, Cleanliness, etc
Problem-free gasoline storage, distribution, and vehicle/engine operation requires a practical freedom from water, acids, and foreign matter particles. The gasoline must have long term and high temperature stability in the presence of air, (oxygen) and must be neutral to metals and elastomers. A number of laboratory tests with specified maximum tolerable limits are used to define these properties (see Chap. 6). Some of these properties can be influenced by additives (see Chap. 5), e.g., water shedding, anticorrosion properties, oxidation stability. The long-term effect of fuel on elastomers in the fuel system is controlled by the selection of suitable materials by fuel system manufacturers. These materials have been tested in standardized and critical reference fluids. 3.1.5. Performance Additives
To ensure safe operation of the engine, highquality gasolines contain special additives which ensure optimum inlet system cleanliness (see Section 5.1.2) and prevent corrosion (corrosion inhibitors, see Section 5.1.1).
3.2. Quality Aspects of Diesel Fuels The diesel combustion process requires a fuel, which in contrast to gasoline ignites easily.
434
Automotive Fuels
Ignition quality is therefore the main quality criterion of diesel fuel. Density is important as an indicator for fuel consumption. Cold flow performance, fuel lubricity, viscosity, stability, and cleanliness control mainly reliability of engine operation. All the above mentioned aspects together with sulfur content have effects on emissions. Fuel volatility is far less important than that of gasoline.
3.2.1. Ignition Quality
Since the fuel is not ignited by an external source in the diesel engine, good ignition performance of diesel fuel is an important criterion for its quality. Ignition quality is determined by characteristics of the components used in production of the fuel (see Section 4.2) and can be improved by additives (ignition improvers, cetane improvers, see Section 5.2.1). The cetane number is used to describe the ignition quality of diesel fuels. Like the octane number, the cetane number is determined by comparative measurements in single-cylinder test engines (CFR swirl chamber engine (EN ISO 5165, ASTM D 613), or BASF prechamber engine) [21]. Mixtures of a-methylnaphthalene with very low ignition quality (cetane number 0) and cetane ( n-hexadecane) with very high ignition quality (cetane number 100) are used as references. The cetane number of a reference mixture is given by the volume percentage of cetane in a-methylnaphthalene. For the determination of ignition quality the ignition delay (time interval between start of injection and start of self-ignition) is kept constant by varying the compression ratio of the engine depending on the test fuel quality. The ignition quality of diesel fuels can also be calculated using the cetane index method (EN ISO 4264, ASTM D 4737). However the cetane index gives only an indication of the ignition quality of a diesel fuel without cetane improvers. This index is calculated by means of a complex formula based on density and distillation range. The formula is updated in regular intervals depending on changes in diesel fuel quality and production technology. A high cetane number is advantageous for the ignition and starting behavior, the reduction of exhaust gasandnoiseemission (‘‘clatter’’) [22,23].
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3.2.2. Density
The density of a diesel fuel (EN ISO 12185, ASTM D 4052) has a considerable effect on the engine performance. Because the quantity of fuel injected into an engine is metered by volume, the mass of fuel introduced into the engine increases with density. A higher fuel density leads to an enrichment of the fuel – air mixture which in principle yields a higher engine power output. On the other hand a high density increases also particulate emissions and black smoke [24]. The maximum density for diesel fuels was therefore 3 reduced from 860 to 845 kg/m in Europe.
3.2.3. Sulfur Content
Exhaust emissions, specifically particulates, are increased by the sulfur in the diesel fuel. In addition acidic combustion products arising from sulfur (sulfur oxides) can lead to engine corrosion. In Europe the maximum sulfur content had therefore been limited to 350 ppm from January 1, 2000 onward and was further reduced to 50 ppm by 2005. Furthermore, it was decided to make ‘‘sulfur free’’ diesel fuels (sulfur contents less than 10 ppm) generally available to allow the use of improved exhaust gas after treatment systems. A tax incentive in Germany for sulfur free diesel fuels led to a widespread availability of these fuels.
3.2.4. Cold Flow Properties
The composition of a diesel fuel affects its filterability at low temperature to a great degree. Particularly, n -paraffins with high ignition quality tend to form wax crystals at low temperature, which can lead to clogging of the fuel filter. For this reason during winter periods the proportion of low boiling components such as kerosene and others with reduced concentration of n -paraffins is often increased in relation to higher n-paraffincontaining gas-oil fractions. Diesel fuels with filterability at very low temperatures, such as arctic fuels, consequently may have lower densities, viscosities, and also ignition performance than normal diesel fuels. The low-temperature filterability of diesel fuel has been substantially improved since the
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1970s by use of highly active flow-improving additives at reasonable costs [25]. The cold filter plugging point (CFPP), a laboratory test EN 116, gives an indication of the low-temperature behavior of diesel fuels (see Section 5.2.3). The cold flow performance of fuel components is also characterized by the cloud point, EN 23015, i.e. the temperature at which wax crystals begin to form. For fuels containing cold flow additives the cloud point has little relevance. However, it is used for the specification of arctic fuels as a safeguard with values 10 C above the CFPP filterability.
3.2.5. Lubricity
The introduction of low and ultra-low sulfur fuels initially led to severe problems due to wear in fuel injection pumps, because the desulfurization process removes substantial amounts of molecules acting as natural lubricants. These have to be substituted by highly efficient lubricity additives (see Section 5.2). The lubricity performance of diesel fuels is defined by a laboratory rig test according to EN ISO 12156-1.
3.2.6. Viscosity
To ensure optimal engine performance the fuel’s viscosity at 40 C has to comply with narrow limits, e.g., in Europe it must be between 2.0 and 4.5 mm2 /s. (EN ISO 3104). Too low a viscosity can lead to wear in the injection pump; too high a viscosity deteriorates injection and mixture formation.
3.2.7. Volatility
The diesel engine is far less sensitive to changes in the fuel volatility than the Otto engine. However, the boiling range of a diesel fuel (EN ISO 3405) influences other properties such as viscosity, flash point, autoignition temperature, cetane number, density, and cold flow performance. Raising the back end temperature, for example, also raises the cloud point; lowering the temperature of the front end also lowers the flash point of the fuel. Generally speaking the variations in volatility of commercial diesel fuels are modest
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435
and usually far less significant than the influence of individual engine design features. To reduce exhaust emissions the amount of high boiling fuel components has been limited by lowering the specified 95% point (temperature for 95% evaporation) from previously 370 C to 360 C. Because of the intercorrelation with the other above mentioned physical properties it is not necessary to specify the light end of the diesel fuel by distillation properties.
3.2.8. Diesel Fuel Stability, Cleanliness, etc
Safe storage and transportation of diesel fuels, their filterability, injector cleanliness, and compatibility with metals is achieved by meeting a number of specification criteria: The minimum flash point (EN ISO 2719) of diesel fuel has been set to 55 C in Europe bylaw. This limit has been adapted to the safety design elements of diesel fuel tanks which are less rigid than those for gasoline. Contamination of diesel fuel with small proportions of gasoline reduces the flash point below the legal limit and makes storage and transportation unsafe. However, it is legal to blend diesel fuel with gasoline in the vehicle tank to improve the cold flow performance at extremely low temperatures. Total dirt (EN 12662) and water content (EN ISO 12937) are limited to guarantee acceptable filterability especially at low temperatures when filter elements may additionally be loaded with wax formed from paraffinic fuel components. Carbonresidue(EN ISO 10370)andoxidation stability(EN ISO 12205,ASTM D 2274)givean indication of possible deposit formation in thefuel system. Deposit formation is also controlled by addition of additives (see Section 5.2.2). The anticorrosion properties are specified by means of the copper corrosion test (EN ISO 2160). Again they can be improved by specific additives (see Section 5.2.6).
3.2.9. Diesel Fuel Effects on Exhaust Emissions
The performance of a diesel engine is less sensitive to changes in fuel quality than that of the gasoline engine. However, in the diesel engine the fuel effects on emissions are more complex
436
Automotive Fuels
for several reasons: (1) The reduction of undesired exhaust gas constituents was so far achieved mainly during the combustion process and to a lesser extent by aftertreatment; (2) there is a wider range of engine technology in use (however there is now more conformity, i.e., direct injection engines with electronically controlled high pressure injection systems); (3) the diesel fuel composition is more complex than that of gasoline, i.e., because of the higher boiling range the individual molecules are less well defined (e.g., monoaromatics are mainly chemically bonded to long chain paraffins). Also properties and composition are often interlinked, such as aromatics concentration and density. The effect of fuel properties and composition on exhaust emissions has been studied by oil companies and motor manufacturers – partly in joint programs, e.g., the European Program on Emissions, Fuels and Engine Technologies (EPEFE). The effect of some fuel parameters, i.e., sulfur and monoaromatics content had been sufficiently documented previously. Sulfur in the fuel increases the emission of sulfur dioxide and of exhaust gas particulates, it has also negative effects on aftertreatment systems. Monoaromatics in the fuel have no measurable effect on exhaust emissions. The effect of other important fuel properties, i.e., density, polyaromatics content, cetane number, and final boiling point has been studied in the EPEFE program. The results of this investigation are summarized in Tables 3 and 4 [20]. For overall reduction of unwanted exhaust gas components it was decided to tighten the specification of diesel fuels in two steps in the years 2000 and 2005, (see also Chap. 6) by increasing the cetane number, reducing the maximum sulfur content, the final boiling point and the density and by introducing a maximum limit for polyaromatics.
3.2.10. Performance Additives
To ensure optimum operation of the car or truck, high-quality diesel fuels contain a variety of special additives which ensure maximum cleanliness of the engine (detergents), minimize the foaming of the diesel during fueling processes (antifoaming additives), prevent corrosion
Vol. 4 Table 3. Summary of effects of diesel fuel properties on emissions in a light duty vehicles (Source: EPEFE program)
Reduce sulfur Reduce densityb Reduce b polyaromatics Increase cetane number d Reduce T 95
c
Change
CO
HC
NO x
PM
2000 to 500 ppm 850 to 820 kg/m3 6 to 3 vol%
0
0
0
!
!
!
0
!!!
50 to 55
!!
!!
0
!
370 to 330 C
0
0
0
0
0 ¼ no effect. 0 ¼ 2 to þ 2%. ! or ~ ¼ 2 – 10% effect. !! or ~~ ¼ 10 – 20% effect. !!! or ~~~ ¼ > 20%. ? ¼ insufficient information. a Effects are not necessarily additive. b Insufficient data is available to reliably separate the influence of these parameters. c PM ¼ particulate matter. d T 95 ¼ temperature for 95% evaporation.
Table 4. Summary of effects of diesel fuel properties on emissions in a heavy duty vehicles (Source: EPEFE program)
Reduce sulfur Reduce densityb Reduce b polyaromatics Increase cetane number d Reduce T 95
c
Change
CO
HC
NO x
PM
2000 to 500 ppm 850 to 820 kg/m3 6 to 3 vol%
0 0
0 0
0
!!
!
!
50 to 55
!
!
!
!
370 to 330 C
0
0
0
0
0 ¼ no effect. 0 ¼ 2 to þ 2%. ! or ~ ¼ 2 – 10% effect. !! or ~~ ¼ 10 – 20% effect. !!! or ~~~ ¼ > 20%. ? ¼ insufficient information. a Effects are not necessarily additive. b Insufficient data is available to reliably separate the influence of these parameters. c PM ¼ particulate matter. d T 95 ¼ temperature for 95% evaporation.
(corrosion inhibitors), improve lubricity (lubricity improver) and improve the cold flow performance (flow improver, wax antisettling additives). Also ignition improvers may be used (see Section 5.2).
4. Fuel Components The minimum requirements for fuel quality are defined in standards or specifications (see Chap. 6) to guarantee trouble-free engine operation and acceptable exhaust emissions. These requirements must be fulfilled in refineries by
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best use of the available product streams [26]. The blending of gasoline and diesel fuels therefore requests the careful selection of suitable refinery components, with sometimes very different physical and chemical properties in different refineries (see also ! Oil Refining) or as a result of crude oil properties. More stringent requirements for the improvement of exhaust emissions post catalyst bring with them more and more intense refining processes such as, e.g., desulfurization.
4.1. Gasoline Components The most important criteria that determine the suitability of gasoline components are the octane numbers (RON and MON), volatility (boiling properties and vapor pressure) and more recently chemical composition (aromatics, absence of benzene, olefins and sulfur) (Table 5). Gasoline fuels consist of hundreds of individual chemical substances belonging mainly to one of the following three groups: aromatics, paraffins, and olefins (Table 6). Oil refineries normally have extremely interdependent structures (! Oil Refining) and, accordingly, many different fuel components are produced. The properties of fuel components, however, depend not only on the type of conversion process and the feed used, but also substantially on the mode of operation of the process.
4.1.1. Straight-Run Gasoline
This gasoline component is obtained by distillation and is therefore easiest to produce and also longest in use. The composition of straight-run gasoline depends on the type of crude oil used (frequently high paraffin content). The octane quality is usually low. In the past, when highly leaded fuels were used, straight-run gasoline was a primary component of fuel. Currently (ca. 2000), the proportion of straight-run gasoline in motor fuel is generally very low (0 – 20%). Butane also belongs to the straight-run products but it has a high octane level. It is added to gasoline, besides being used as liquefied petroleum gas (! Liquefied Petroleum Gas). Because of its high vapor pressure, butane is good for
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Table 5. Important properties of various gasoline components (approximate values) [3] a
Component Straight-run gasoline Butane Pyrolysis d gasoline Light coker gasoline Light catalytically cracked gasoline Heavy catalytically cracked gasoline Light hydrocracked gasoline FR reformate (94) e FR reformate (99) f FR reformate g (101) Isomerizate (isopentane) Alkylate Polymer gasoline Methyl tert -butyl ether Methanol – tert butanol (1 :1)
r15,
g/mL MON
RON
E 70, vol%
E 100, vol%
0.680 0.595 0.800
62 64 70 c 87 – 94 92 – 99 100 82 97 35
100 100 40
0.670
69
81
70
100
0.685
80
92
60
90
0.800
77
86
0
5
0.670 0.780
84 84
90 94
70 10
100 40
0.800
88
99
8
35
0.820
89
101
6
20
0.625 0.700 0.740
87 90 80
92 92 100
100 15 5
100 45 10
0.745
98
114
100
100
0.790
95
115
50
100
b
a
E 70 ¼ amount of fuel components with bp < 70 C. b E 100 ¼ amount of fuel components with bp < 100 C. c Depending on the isobutane content. d BTX aromatics extracted. e Full range platformate with RON ¼ 94. f Full range platformate with RON ¼ 99. g Full range platformate with RON ¼ 101.
engine cold-start. On the other hand, too high a vapor pressure can lead to hot-fuel handling problems (warm start problems, stumbling engine, vapor lock) and increased evaporative hydrocarbon emissions [27]. Thus, addition of butane is limited by the vapor pressure specifications. The yield of straight-run products supplied naturally by crude petroleum cannot meet the high demand for gasoline either qualitatively or quantitatively. Thus, high-boiling components of crude petroleum are cracked either thermally or catalytically into lower boiling products that can be used in gasoline production.
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Table 6. Approximate composition of various fuel components (in volume percent)
Component
Paraffins
Olefins
Aromatics
Straight-run gasoline Butane Pyrolysis gasoline* Light coker gasoline Light catalytically cracked gasoline Heavy catalytically cracked gasoline Light hydrocracked gasoline FR reformate (94) ** ** FR reformate (99) FR reformate (101) ** Isomerizate Alkylate Polymer gasoline
94 100 ca. 20 ca. 57
1
5
ca. 10 ca. 40
ca. 70 ca. 3
61
26
13
29
19
52
100 45 38 29 98 100 5
0
0 55 62 70 2
*
1
90
5
tionislimited.Thehigholefincontentofthesefuel componentsalsorequires specialprecautionsto be takenagainstpolymerizationandoxidationduring their use (see Section 5.1.3). Another variant of catalytic cracking is hydrocracking, a process in which hydrogen is added. The hydrocracked gasoline produced by this process contains practically no unsaturated components and consists principally of iso- and cycloparaffins. Hydrocracked gasoline has a high MON, a moderate RON, relatively low density, and high volatility [29].
4.1.4. Catalytic Reformate (Platformate) (! Oil Refining, Section 3.4.)
Reflux from petrochemical plants; after partial hydrogenation. For abbreviations, see Table 5.
One of the most important motor gasoline component is catalytic reformate, sometimes referred to as platformate. Reformate has an exceptional4.1.2. Thermally Cracked Gasoline ly high aromatic content and a very high octane quality. It is obtained by reforming (isomerizaThermallycrackedgasolineisoftenusedintheform tion and dehydrogenation reactions) from parafof pyrolysis gasoline. It is obtainedin particular as a fin-rich straight-run gasoline with a low octane byproduct from steam crackers (! Ethylene) in quality [30]. The properties of the reformate petrochemical plants and has a high content of essentially depend on the mode of operation in olefins and aromatics (with good octane quality). the reforming process [31]. When very high BTX (benzene, toluene, xylene) aromatics are fre- octane numbers are achieved, the aromatics conquently extracted from pyrolysis gasoline as valu- tent and the density increase, but at the same time able raw materials for chemical processes, so only the yield and volatility decrease. Because of its the residues from petrochemical plants are some- high octane quality, reformate is the backbone for times used in gasoline production [28]. the production of lead-free premium fuels. The Variants of thermal cracking are visbreaking restrictions of benzene and aromatic content in and coking (! Oil Refining, Section 3.6.4.1.). gasoline fuels limit the use of these components The relatively small quantities of gasoline ob- and also requires specific treatment of the retained (visbreaker gasoline and coker gasoline) former feed and/or the reformed product to reare olefin-rich and have a low octane quality. move the benzene from this stream (mainly distillation and hydrogenation techniques). **
4.1.3. Catalytically Cracked Gasoline
Catalytically cracked gasoline is more important forgasolineproduction.Ithasahighconcentration of olefins and has to be desulfurized to be suitable for modern gasolines. Catalytically cracked gasolineis usuallyseparated intohigh-boilingandlowboiling fractions, which have different applications. The octane quality (especially the MON) of catalytically cracked gasoline is usually sufficient only for regular gasoline. In order to meet the legally defined olefin specification of gasolines the amount which can be used in gasoline produc-
4.1.5. Isomerate (! Oil Refining, Section 3.7.3.)
The demand for volatile gasoline components with sufficient octane quality cannot be met by, e.g., unrefined straight-run gasoline [32]. For this reason, from the 1970s onward the low-octane straight-run gasoline was increasingly converted into the higher octane isomerate (mainly isoparaffins). Because it is free from aromatics isomerate is becoming increasingly attractive for environmental reasons.
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4.1.6. Alkylate (! Oil Refining, Section 3.7.1.)
Another attractive fuel component consisting mainly of isoparaffins is alkylate, which is moderately high in octane quality (especially with regard to the MON), however, less suitable to formulate higher octane premium fuels like Super Plus (minimum RON 98). Alkylate is obtained by the reaction of isobutane with olefins and can only be produced in limited capacity because of the low availability of isobutane. Since the introduction of mandatory maximum aromatic levels in gasolines alkylate has becomeoneofthemostimportantcomponentsfor the production of premium fuels. Alkylate is the main component in aviation gasoline (Avgas). 4.1.7. Polymer Gasoline (! Oil Refining, Section 3.7.2.)
A less common gasoline component is polymer gasoline, obtained by oligomerization of propene or butenes produced in cracking plants. It consists primarily of C6 – C12 olefins. 4.1.8. Oxygenates
High octane quality of unleaded gasoline can be obtained more easily by the addition of oxygencontaining fuel components [33, 34]. In particular, the addition of methyl tert -butyl ether (MTBE) (! Methyl tert-Butyl Ether) has proved to be very useful for maintaining the octane quality of unleaded high octane fuels, even more since the introduction of maximum aromatic levels of 42 vol% (2000) and 35 vol% (2005). Besides its high octane quality MTBE is a very clean burning gasoline component. Furthermore, methanol, ethanol produced from biomass, or higher alcohols can be used in low concentrations.
4.2. Diesel Fuel Components The self-igniting diesel combustion process requires molecules with different structures than used in gasoline, i.e., in contrast to gasoline diesel fuel components must decompose easily at high temperature and pressure. Normal n-paraffins are
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Table 7. Properties of various hydrocarbon groups with regard to their suitability as diesel fuels
Ignition Cold flow Volumetric Density Smoking quality properties calorific value tendency n-Paraffins good poor Isoparaffins low good Olefins low good Naphthenes moderate good Aromatics poor moderate
low low low moderate high
low low low low low moderate moderate moderate high high
for that reason ideal diesel fuel constituents, in contrast to olefins and aromatics. However, becauseofthehigherboilingrangeofdieselfuel,the hydrocarbon molecules often consist of a combination of different structural moieties (e.g., an aromaticringwithalongparaffinicorolefinicside chain). The main characteristics determined by the composition of the diesel fuel are its ignition performance, cold flow properties, volumetric calorific value, and density. The latter can affect the air – fuel ratio and therefore influences particulate emissions (smoking tendency) under maximum torque conditions. None of the classes of substances present in diesel fuel fulfills all the criteria equally well; for example, n-paraffins, which have a very good ignition performance and low smoking tendency, show poor low-temperature behavior and have a low calorific value (Table 7). However the combination of, e.g., paraffinic and aromatic substructures in the molecules can reduce cold flow problems without significantly impairing the combustion properties. The diesel engine can also theoretically be run on lower and higher boiling hydrocarbons. Marine diesel engines for example use higher boiling residual fuels (! Heating Oil, Chap. 2.). The particular value of the automotive diesel engine lies, however, in its ability to utilize economically the middle distillates in the distillation range 150 – 360 C. Accordingly, the diesel fuel components produced in refineries must correspond to the aforementioned requirements as far as possible.
4.2.1. Straight-Run Middle Distillate
Straight-run middle distillate (distillate gas oil) represents a valuable diesel fuel component
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are present in excess and are, therefore, converted into lighter products; (3) the refinery structures also provide a certain ‘‘compulsory fraction’’ of middle distillates from conversion processes; and (4) particular quality requirements can also make the production of special diesel fuel components necessary.
4.2.2. Thermally Cracked Gas Oil
Figure 4. Typical properties of gas-oil components a) Synthetic middle distillate; b) Straight-run gas oil; c) Hydrocracked gas oil; d) Thermally cracked gas oil; e) Syncrude gas oil; f ) Coker gas oil; g) Light catalytically cracked cycle oil
because of its usually high cetane number (see Fig. 4). The quality of straight-run middle distillate nevertheless depends strongly on the quality of crude petroleum used. In many cases, distillate gas oil has a relatively high paraffin and low aromatic content. Although middle distillates can be used for the diesel engine without further processing (apart from desulfurization), other diesel fuel components are also employed. The following reasons are decisive: (1) the natural middle distillate fraction in crude petroleum is generally not sufficient to meet the demand for middle distillate; (2) unwanted heavy components in crudes
Thermally cracked gas oil originates from either the visbreaking or the coking process and tends to have a lower cetane number and higher density than distillate gas oil (Table 8). Thermally cracked gas oil usually goes through a subsequent hydrogenation step because of the formation of unstable olefinic components during the cracking process. After hydrogenation, it represents a usable diesel fuel component.
4.2.3. Catalytically Cracked Gas Oil
Catalytically cracked gas oil is generally characterized by a very low cetane number and high density. Because of its instability, catalytically cracked gas oil must normally be hydrogenated [35]. Because of the poor ignition performance resulting from the high aromatic content and the
Table 8. Properties of diesel fuel components
Process
Component
Composition Before treatment
After treatment
Distillation
distillate gas oil (straight run)
content of aromatics: medium; olefins: very low; paraffins: high
Thermal cracking
thermocracked gas oil
content of aromatics: low; olefins: medium; paraffins: high
content of aromatics: low; olefins: none; paraffins: high
Catalytic cracking
catalytically cracked gas oil
content of aromatics: high; olefins: low; paraffins: low
content of aromatics: high; olefins: none; paraffins: low
Hydrocracking
hydrocracked gas oil
content of aromatics: very low; olefins: none; paraffins: very high
SMDS synthesis
synthetic gas oil
content of aromatics: none; olefins: none; paraffins: very high
Distillation or hydrocracking
kerosene
content of aromatics: low; olefins: none; paraffins: very high
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high smoking tendency, it can be added to diesel fuel only in limited quantities.
3. Isomerization to isoparaffins 4. Distillative separation into kerosene and diesel fraction
4.2.4. Hydrocracked Gas Oil
The product was initially used mainly for improvement of the poor quality of the straightrun middle distillate otherwise available in Malaysia. The middle distillate produced by the SMDSprocessresultsinlowerexhaustemissionin diesel engines because it hasa high cetane number (> 60). To meet increasing requirements for diesel fuel quality of modern low emission engines synthetic middle distillates are meanwhile used as components in top-quality diesel fuels. A process is currently under development is to convert biomass to gas on a commercial scale with subsequent use of the SMDS process.
Hydrocracked gas oil is a very valuable diesel fuel component. Because of its very high paraffins content, the ignition performance is good and the smoking tendency low.
4.2.5. Kerosene
Improved cold flow properties of diesel fuel can be achieved by the addition of kerosene. Because kerosene is predominantly used as jet fuel and, therefore, expensive and in short supply, its addition is limited mainly to meet the requirements of low temperature climatic conditions. The density, calorific value, and viscosity of kerosene are in most cases, however, much lower than those of other diesel fuel components and therefore its use is limited anyway. The low density and low calorific value would lead to decreased engine power and increased volumetric fuel consumption. The low viscosity would require additives to avoid wear in the injection pump because of poor lubricity.
4.2.6. Synthetic Diesel Fuel
Diesel fuel can also be obtained from fossil energy sources other than crude petroleum. During World War II diesel fuel was produced from coal by the Fischer –Tropsch process in Germany. This synthetic fuel (kogasin) had a range of outstanding properties: cetane number ca. 100, almost sulfur-free composition, low density, and high gravimetric calorific value. This synthetic diesel fuel component was used principally for enhancing low-value coal tar oil. Synthetic diesel fuel is produced in small quantities by the Shell Middle Distillate Synthesis (SMDS) process in Malaysia from natural gas [36]. The process includes the following steps: 1. Steam reforming of natural gas (conversion to synthesis gas), 2. Buildup of long-chain n-paraffins (Fischer – Tropsch synthesis),
5. Fuel Additives By the generally accepted definition only agents that are added to fuels in a concentration of < 1% should be described as ‘‘additives.’’ For substances present in higher concentration, the term ‘‘fuel components’’ is more appropriate. Fuels are treated with additives for various reasons: in the past the addition of octane enhancers to gasolines enabled the production of high octane grades. Nowadays the use of cetane improvers offers the possibility of upgrading a base fuel very economically compared with refinery measures. Performance additives, on the other hand, which are added to modern high quality fuels, improve the behavior of a base fuel in operation, and offer technical advantages that often cannot be achieved by measures taken in the refinery [37]. In many cases, these types of additives offer the only possibility of guaranteeing trouble-free engine performance over a longer running period. Finally, treatment of fuels with additives is a major route to achieve product differentiation and trademarked quality. Thus it is not surprising that in Western Europe further increase in the use of additives is generally anticipated [38] with the exception of the alkyllead compounds which due to their incompatibility with exhaust catalysts and their toxicity have been phased out as of January 1, 2000 in the EU. A derogation of two years for the lead phase out was granted to Spain, Italy, and
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Greece by the EU commission. Lead phase out is a general trend worldwide, except for developing countries with low car population.
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specifications as well as demanding new technologies like direct injected gasoline cars.
5.1.1. Corrosion Inhibitors
5.1. Gasoline Additives The treatment of gasoline with additives is almost as old as the fuel itself [39]. At first (around 1900), the search for so-called brisance-increasing additiveswaspredominant(Fig. 5).In1918,treatment with certain additives was recognized to allow an increase of compression ratio without dangerous knocking. An increase in the octane number became the major aim for decades in additive technology. Systematic research into additives that could positively influence many aspects of engine behavior began around 1950 on a wider scale. Since ca. 1970, conventional, ash-forming antiknock additives became increasingly less important for environmental reasons. Nowadays additive development is driven by environmental
Figure 5. Historical development of gasoline additives
Water is almost always present in traces and can ingress into the fuel during processing, transfer, and storage by condensation. Moreover alcohols, when present in the fuel, can also lead to corrosion problems either by themselves or because of their water-solubility. Corrosion must be inhibited not only to avoid damage of the tank or the fuel system but also because fine rust particulates can block fuel filters and injectors. The effect of corrosion inhibitors is based on their ability to cover a metal surface with a monomolecular layer of inhibitor molecules and thus protect it from contact with corrosive components [40]. The structure of the corrosion inhibitors corresponds to this requirement: a polar end of the molecule (e.g., a carboxylate,
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Figure 6. Mode of action of corrosion inhibitors
ester, or amine group) is attached to the metal surface; while a nonpolar, high molecular mass alkyl chain at the other end of the molecule extends into the hydrocarbon phase (Fig. 6). An oily covering layer is thus formed that effectively repels water. High alcohol contents in fuel (e.g., gasohol in Brazil)areaproblembecausealcohols,inadditionto their own corrosiveness, can also dissolve the protective film. Apart from these cases, which require exceptionally high concentrations (e.g., 100 ppm) of corrosion inhibitors, 10 – 20 ppm, is generally sufficient for effective corrosion protection. The efficiency of corrosion inhibitors can be tested in the corrosion test according to DIN 51585, ASTM D 665, in which gasoline – water mixtures are allowed to react with steel test objects. Examples of suppliers are BASF (Kerokorr additives), Octel (DCI series), Lubrizol (LZ series), Bayer AG (RC additives), Infineum, Nalco/ Exxon, Baker/Petrolite (Tolad) Betz-Dearborn, Apollo Technologies (PRI), UOP (Unicor), Ethyl (Hitec), and Champion Technologies (RPS).
5.1.2. Detergents
In Otto engines, high molecular mass organic deposits can be formed in various sites in the fuelling and inlet and mixing systems. These deposits can lead to considerable problems in engine operation. Deposits on the carburetor and injectors prevent the formation of an optimal fuel – air mixture and cause increased fuel consumption and poor exhaust gas emissions [41–43]. In extreme cases the engine can even stall. Severe deposit formation on the inlet mani folds and valves causes, in addition to increased fuel consumption and deterioration in exhaust emissions, a decrease in power [44–47]. Sometimes even mechanical engine damage can occur.
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The formation of deposits is caused by several factors [48–51]: Fuel components with high olefin or heteroatom content (which also have a marked tendency toward gum formation) are relatively prone to deposit formation. This is also true for fuels with very high final boiling points. Engine oil, which enters the inlet system via the valve stem guides, also contributes to deposit formation. Finally, combustion gases play a role in deposit buildup. These gases get into the inlet system either with the recirculated blowby gases from the crankcase or directly from the combustion chamber because of valve overlap. The multiplicity of factors contributing to deposit buildup shows that the problem cannot be solved by individual improvements in single areas but only by the addition of detergents with extensive effectiveness. Detergents for Carburetor and Injector Cleanliness. Detergent additives were first used in the 1950s to maintain carburetor cleanliness (‘‘keep clean effect’’). Later, detergent additives were developed which removed deposits already present in the carburetor (‘‘cleanup effect’’). First-generation detergents were specially designed to work in the relatively cold region of the carburetor. These detergents include alkylamines, alkyl phosphates, alkylsubstituted succinimides, imidazolines, and oleylamides, which were added to the fuel in concentrations of 50 – 100 ppm. These compounds are also suitable, in principle, for keeping injectors clean, although concentrations of 150 – 300 ppm are then required [52]. To assess the efficiency of a carburetor detergent additive, the CEC developed a test cycle with a Renault 5 engine [53]. Detergents for Inlet Valve Cleanliness. From the mid-1970s, the search for decreasing fuel consumption and increased power led to leaner fuel – air mixtures and temperature increases in the inlet system. This created several problems that could not be overcome by the firstgeneration detergent additives, because these were effective only in the low-temperature range. In individual cases the carburetor detergents caused even higher deposit accumulation on the inlet valves due to decomposition. The development of second-generation detergent additives made it also possible to keep the
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hot inlet valve clean [47], [54–56]. Long-chain, high molecular mass alkyl compounds (molecular mass around 500 – 1500 g/mol) with a polar end group, are very effective. The mechanism is analogous to that of corrosion inhibitors described in Section 5.1.1. Effective, modern detergent additives are based on polyisobuteneamine, polyisobuteneamide/succinimide, or polyetheramine technology. To meet most stringent environmental legislation these additives have to be produced halogen-free. These detergents, except for the polyetheramines, are normally used in combination with very thermally stable carrier oils that keep the surface of the valve lubricated and ensure continuous flowing off of deposit particles. To keep inlet valves clean relatively high concentrations of detergent (300 – 400 ppm) and carrier oil (300 – 400 ppm) are used. Depending on the polar end group modern detergents provide both keepclean as well as cleanup performance. A number of tests have been developed to assess the detergency performance of these gasoline additives: the Mercedes Benz M102E test was developed in Europe by CEC [57], [58]. Examples of suppliers are BASF (Keropur multipurpose additives), Infineum (Vectron), Ethyl (Hitec), Octel (OGP), Oronite (OGA series), and Lubrizol (ADX).
5.1.3. Antioxidants (! Antioxidants)
As early as 1930 the aging of gasoline was known to be inhibited considerably by antioxidants (oxidation inhibitors). Nevertheless, antioxidants achieved importance only after World War II, when olefin-rich fuel components from cracking processes were increasingly used (! Oil Refining). Olefins, in particular conjugated dienes, undergo liquid-phase free-radical chain reactions relatively easily at low temperature, which finally lead to the formation of highly polymeric residues, the so-called gum [59]. Other potentially gum-forming components are aromatics and the nitrogen and sulfur compounds present at trace levels. The efficiency of antioxidants is based on their capability to interfere with the mechanism of gum formation. In this mechanism, the initiating step is formation of a radical (R1H ! R1); in the second step, hydroperoxides are formed by reaction with dissolved
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oxygen from the air and the chain reaction is started (Eqs. 1 and 2): 1
R
1
R
þO2 !R1 O O
ð1Þ
O OþR2 H!R O OHþR2
ð2Þ
Gum-forming reactions continue until the chain reaction is terminated by the coupling of two radicals (Eqs. 3 and 4). The chain termination reaction can also be taken over by an antioxidant (AH), which leads to the formation of an inactive radical A that is not able to continue the chain reaction (Eq. 5): 1
þR2 !R1 R2
ð3Þ
1
O OþR2 !R1 O O R2
ð4Þ
1
O O:þAH!R1 O O HþA
ð5Þ
R
R
R
The aging of gasoline must be inhibited not only because of gum formation, which (1) blocks carburetors and injectors, filters in engines or dispenser pumps; and (2) can lead to dirt accumulation in the inlet system. An additional function of the antioxidant used to be the stabilization of tetraethyllead. The gum content of gasoline fuels is measured according to EN ISO 6246, ASTM D 381, giving the washed gum evaporation residue. The residue has to be washed with n-hexane to remove detergent additives and carrier oils used in high performance fuels. The induction period (EN ISO 7536) is a laboratory test to measure the storage stability of gasolines. Most antioxidants belong to one of the following two classes of substance. para-Phenylenediamines have the general formula R1NH C 6H4NHR2, in which R1 and R2 can be sec-butyl-, isopropyl-, 1,4-dimethylpentyl-, or 1-methylheptyl-. para-Phenylenediamines are very effective at concentrations of 5 – 20 ppm, particularly for gasolines with high olefin content. Their basicity, however, makes them susceptible to reaction with acidic impurities or fuel components. On the other hand, phenylenediamines offer the advantage of catalyzing the oxidation of mercaptans (which have foul odors even when present in trace amounts) to odorless disulfides.
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Hindered Alkylphenols. Alkylphenols are optimally efficient if their hydroxyl group is sterically hindered by alkyl groups in positions 2 and 6, e.g., as in 2,6-di- tert -butylphenol. Alkylphenols are particularly suitable for olefin contents < 10%. Because of steric hindrance, the acidity of the hydroxyl group is lowered to such a degree that the danger of side effects is low. To achieve the same efficiency as phenylenediamines, higher concentrations (5 – 100 ppm) are necessary. Because of their positive synergism, mixtures of phenylenediamines and alkylphenols are also used sometimes [60]. Examples of suppliers are BASF (Kerobit additives), Octel (AO), and Ciba (Irganox).
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Figure 7. Effect of relative air humidity and air temperature on carburetor icing [54] a) Severe icing; b) Icing; c) No icing
point fuel injection systems which are practically immune to icing. During the evaporation of fuel in the carbureThe aging (oxidation) of gasoline can be accel- tor or in single point injection systems, heat is erated considerably by traces of metals that get absorbed from the surroundings so that even at into the fuel during transfer. Mercaptans, phe- ambient temperatures > 0 C, temperatures benols, and alcohols increase the solubility of low 0 C can occur and lead to freezing of the air metals in fuel. Copper has the highest catalytic moisture there. Besides its temperature, the relaactivity, accelerating the oxidation of fuel con- tive humidity of the air is an important factor for siderably at concentrations as low as 0.01 mg/L. the formation of icing (Fig. 7) [54]. The function of metal deactivators is based on Carburetor icing was particularly noticeable the fact that they form stable chelate complexes during idling (engine stalling because of ice with dissolved metal atoms so that the metal loses formation) or constant speed driving making its undesirable catalytic activity [40]. The addi- itself noticeable by engine power loss, high fuel tion of N,N 0 -disalicylidene-1,2-propanediamine consumption, unfavorable exhaust gas emisis particularly effective, sions, and finally, engine stalling. To avoid carburetor icing, two different types of additives were used: (1) surfactants and (2) freezing point depressants (! Antifreezes). 5.1.4. Metal Deactivators
and copper is complexed as follows:
Examples of suppliers are BASF (Keromet additives) and Octel (DMD). 5.1.5. Anti-Icing Additives
The problem of carburetor icing has virtually disappeared because of the introduction of multi-
Surface-Active Substances. Surfactants produce protective films over the metal surface so that ice is prevented from sticking to it. Examples of surface-active compounds are amines, diamines, amides, or glycol esters of fatty acids, which are effective at 10 – 60 ppm. Other surface-active substances added to fuels, such as detergents, act as anti-icings in addition. Freezing Point Depressants. Effective freezing point depressants include alcohols, glycols, dimethylformamide, and other watersoluble polar substances. Depending on the type used they were added in concentrations of 0.02 – 2 vol%.
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5.1.6. Additives for Combating Combustion Chamber Deposits
Mainly when leaded gasolines were used considerable quantities of lead compounds, but also carbon, and engine oil components could deposit in the combustion chamber and on the spark plugs [61–64]. This could lead to knocking combustion and to spark plug fouling problems. Organophosphorus and organoboron compounds were therefore added to fuel at an early stage. These additives do not remove the deposits but are at least able to convert them to materials with high glow temperature and low electrical conductivity (deposit modifiers). The commercial products contained, for example, tricresyl phosphate, triisopropyl phosphite, borazane, or boric acid esters. These additives are no longer on the market because (1) the phase out of lead compounds reduced the problem and (2) phosphorus compounds are incompatible with catalytic converters because of their tendency toward catalyst poisoning. Research was devoted to the development of ash-free additives,whichcounteractthebuildupof combustion chamber deposits and control any increase in the octane number requirement of the engine [octane requirement increase control (ORIC) additives]. These additives have shown some positive effects in test engines. However their effect under actual road conditions seems to be limited. Reduced engine oil consumption, precise fuelmeteringto obtain stoichiometric air-fuel ratios, the shape of modern combustion chambers and of course the use of unleaded gasoline have reduced the problem of combustion chamber deposit formation. Specific additives to reduce these deposits are therefore not in general use.
5.1.7. Valve Seat Recession Protection Additives
Some older cars with ‘‘soft valve seats’’ (e.g., cast iron) relied on leaded gasoline, since lead compounds inhibited corrosion and wear of the valve seats. Unleaded gasoline for older cars can be provided with organic potassium or sodium compoundsas lead replacement. Thesecan be addedto thefinishedfuelbytheendconsumerusingabottle solution. However, valve seat recession occurs predominantly under high-duty (motorway)
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operation. As vintage cars with soft valve seats arenormallynotoperatedundertheseconditionsit is generally not necessary to use these additives. 5.1.8. Antiknock Agents (! Octane Enhancers)
The most important antiknock additives were alkyllead compounds, i.e., tetraethyllead and tetramethyllead. As pointed out the use of these additives was discontinued because of their poisoning effects on the exhaust-gas aftertreatment systems and their toxicity. Other metal-based antiknock additives have either similar unwanted side effects or limited activity. Organic antiknock additives are less effective than ash forming products. Best known are nitrogen-containing aniline derivatives. None of these antiknock additives is used commercially on a larger scale. Instead oxygen-containing blending components, mainly ethers (! Octane Enhancers; ! Methyl Tert&Butyl Ether) are used to increase the octane quality of unleaded gasolines. 5.1.9. Dehazers and Antistatic Additives
Dehazers and antistatic additives are rarely used in gasoline. They are similar to the corresponding types of additives in diesel fuel (see Sections 5.2.7 and 5.2.9).
5.2. Additives for Diesel Fuel Apart from a few exceptions treatment of diesel fuel with additives was introduced much later than that of gasoline. Several reasons existed for the introduction of additives: (1) a significant progress in diesel engine technology occurred from about the mid-1970s onward which led to a greater market share of diesel engines in passenger cars; (2) fleet operators and car owners required improved reliability and comfort; (3) more stringent emission standards required improved diesel quality. 5.2.1. Ignition Improvers (Cetane Improvers)
The cetane number (see Section 3.2) is a measure of the ignition quality of a diesel fuel. Because a
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Figure 9. Improvement of exhaust gas emissions by ignition improvers; average emissions of three passenger cars with pre- and swirl chamber diesel engines [67]
5.2.2. Detergent Additives Figure 8. Responseof different base fuel types to addition of ignition improvers
range of aspects essential to the operation of diesel engines are also connected with the cetane number (cold-start properties, particulate, e.g., white and black smoke emission, noise emission, fuel consumption, engine durability), the increase in cetane number by additives is a valuable means of improving diesel fuel quality [65]. The effect that can be achieved by the use of ignition improvers depends on the base diesel fuel. Unfortunately, base fuels with high natural cetane numbers react better to additive treatment than those with lower cetane numbers, for which an increase in the cetane number is more essential (Fig. 8). In view of constantly increasing efforts to reduce exhaust gas emissions, the fact that even small quantities of ignition improvers can improve the exhaust gas quality of the diesel engine is important (Fig. 9) [66, 67]. The effect of ignition improvers is based on their rapid decomposition, with the formation of free radicals that accelerate the chain reaction of diesel fuel combustion. Suitable compounds are alkyl nitrates, ether nitrates, alkyldiglycol nitrates, and organic peroxides. From a commercial point of view only alkyl nitrates such as 2-ethylhexyl nitrate (EHN) have been successful. With these compounds, the ready cleavage of the RONO2 bond leads to radical formation [69]. Examples of suppliers are Octel (CI additives) and Ethyl (Hitec).
Corresponding to the different construction of the diesel engine compared with the Otto engine, the problem of deposit formation occurs in a different form. In diesel engines the injection system is particularly affected, where gum- or carbonaceous deposits are formed on the injectors. This inhibits the ability of the valve needle to move and the formation of a finely dispersed spray of fuel. The mixing system is thus disturbed, and increased emissions (particulates), higher fuel consumption, and lower engine power, result. A range of substances are suitable as detergents in diesel engines: amines, imidazolines, amides, succinimides, polyalkyl succinimides or amines, and polyetheramines [70, 71]. The following mechanism of action has been proposed: the surface activity of the polar groups ensures the formation of a protective film, whereas the dispersing effect of the polymeric chain prevents agglomeration and deposit formation by smaller particles. In this way a ‘‘keep clean effect’’ can be achieved; however, for already dirty nozzles, a solubilization of formed deposits material is also necessary. The addition of detergents offers other advantages for diesel fuel, such as better storage stability and a certain amount of protection against corrosion [72]. Examples of suppliers are Infineum (Vectron), Octel (OMA), Oronite (ODA series), andLubrizol.
5.2.3. Cold Flow Additives
Diesel fuel has the unfortunate property that crystallization of high molecular mass n-paraffins
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(waxes) can occur at low ambient temperature. These wax crystals can clog the fuel filter and lead toenginestoppageafterashortperiodofoperation. The tendency toward wax separation depends strongly on the composition of the diesel fuel, which is determined mainly by the crude petroleum used and the refinery structure [73]. In principle,thelow-temperaturebehaviorofadieselfuel could be improved by removing the particularly critical n-paraffins. However, this would be unacceptablenotonly because ofthe high costs,butalso because n -paraffins are very valuable diesel fuel components due to their good ignition performance and low smoking tendency [74]. Before the development of highly effective additives for improving low-temperature properties, owners of diesel vehicles were frequently forced to add up to 30% regular gasoline to diesel fuel, risking engine damage and increased exhaust emissions. To assess the low-temperature behavior of diesel fuel the following two laboratory test procedures are used:
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a cost- effective way. Refineries are given the flexibility to use high cetane quality paraffinic product streams for diesel fuel, kerosene, otherwise necessary for achieving good low-temperature behavior of diesel fuel, for the production of jet fuel. Flow improvers have been used increasingly since 1973. Their mechanism of action is based on two different effects [77]. First, the additive forms crystallization nuclei on reaching the cloud point, to which the wax crystals become adsorbed. Second, further crystal growth is slowed down by adsorption of the additive on the growing crystal surface. Thus, both the crystal size and the tendency to agglomerate are reduced (Fig. 10). The small crystals thus formed have a considerably lower tendency to block fuel filters because they (1) pass through the filters to some extent, (2) produce a still penetrable layer on the filter, and (3) dissolve again rapidly when the engine is warmed. Flow improvers are effective only when they have been added to the fuel at temperatures well above the cloud point of the base fuel, i.e., 1. Determination of the Cloud Point at about þ 40 C, and preferably in the refinery. (EN 23015, ASTM D 2500). The cloud point Addition after crystal formation is not effective isthetemperatureatwhichdieselfuelbeginsto [78]. Ash-free polymers are used as flow imbecometurbidatadefinedcoolingratebecause provers, in the vast majority of cases ethylene – of the separation of small wax crystals. The vinyl acetate (EVA) copolymers. The dosage cloud point is especially important for the with which a particular CFPP value can be evaluation of refinery components in order to achieved depends on the base fuel and is normaldetermine type and amount of cold flow addi- ly 50 – 500 ppm (Fig. 11). tivethathastobeaddedtothecomponents.The Examples of suppliers are BASF (Keroflux cloudpointdoesnotdefinetheoperabilityofthe additives), Octel (OFI), Infineum, and Clariant final, additivated diesel fuel. (Dodiflow). 2. Determination of the Cold Filter Plugging Point (CFPP) (EN 116) [75, 76]. The CFPP is Wax Antisettling Additives (WASA). the limiting temperature at which, under de- Wax antisettling additives keep small wax crysfined conditions, a diesel fuel sample can no tals in suspension and thus inhibit the formation longer pass through a metal sieve within a of wax layers, which can rapidly lead to filter specified length of time. Generally speaking clogging [25]. This is achieved by their ability to the CFPP gives a reliable indication of the slow the growth of wax crystals, which is even operability limit expected for a diesel fuel in greater than that of flow improvers. According to modern vehicles. Since the development of Stokes’ law, small particles have a considerably wax antisettling additives, which drastically lower settling rate than large ones. By reducing lower the CFPP, the operability of critical the size of wax crystals by one-fifth, the sediolder vehicles (e.g., without heating of the mentation time can be extended from an hour to a fuel system) is not as low as suggested by the day. Wax antisettling additives have been used CFPP result. since 1986 – generally in combination with flow improvers. Typical concentrations are 100 – Flow Improvers. Flow improver additives 500 ppm. They are usually based on modified improve the cold flow properties of diesel fuels in ethylene – vinyl acetate copolymers.
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Figure 10. Mode of action of flow improvers [79] MDFI ¼ Middle distillate flow improvers
Examples of suppliers are BASF (Keroflux ES additives) and Infineum [79]. Cloud Point Depressants. The use of conventional flowimprovers to lower theCFPP value
generally has hardly any influence on the cloud point. However, polymer additives (based on ethylene – vinyl acetate, unsaturated esters, imides, or olefins) have been developed that permit additionalloweringof thecloud point.Theirmode of action is based on shifting the thermodynamic equilibrium by irreversibly decreasing the crystallization temperature. This particularly increases the solubility of higher molecular mass n-paraffins. With a dosage of 500 ppm the cloud point can be lowered by ca. 3 C. The market importance of cloud point depressants is considerablysmallerthanthatofflowimproversandwax antisettling additives [73].
5.2.4. Lubricity Additives Figure 11. Effect of a flow improver on various base diesel fuels [54] a) Poorest response behavior; b) Normal response behavior; c) Best response behavior
The intrinsic, natural lubricity of diesel fuel is reduced during the desulfurization process,
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because surface active components are destroyed during the hydrogenation. In the case of low and ultra-low sulfur diesel fuels the lubricity of the finished product can be too low, resulting in wear problems in the injection pumps. To ensure safe operation low and ultra-low sulfur diesel fuels have to be additivated with efficient lubricity additives. Examples of surface active substances are fatty acid derivatives which are added in the range of 50 – 200 mg/L. The lubricity performance of fuels is evaluated in the high frequency reciprocating wear rig (HFRR) laboratory test (ISO 12156–1). A maximum wear (HFRR value) is defined and specified in EN 590. Examples of suppliers are Octel (OLI), Ethyl (Hitec), Infineum, and Lubrizol (LZ).
than straight-run middle distillates. Middle distillates from cracking processes are generally subjected to hydrogenation (‘‘hydrotreating’’); however, additional treatment with antioxidants is often necessary. In diesel fuel aging, sulfur- and nitrogencontaining substances can also be responsible for sediment formation to a considerable extent, although they sometimes have an oxidationinhibiting effect. In addition, a high acidity (caused by carboxylic acids or phenols) accelerates aging catalytically. Sediment formation brought about by aging can clog the fuel filter; gumlike deposits can affect the flow of the fuel in the injection pump and the injectors. The additives used to inhibit aging are similar to those used for gasoline (see Sections 5.1.3 and 5.1.4), namely sterically hindered phenols and phenylenediamines. Trialkylamines are also very 5.2.5. Antifoam Additives efficient. The additive dosage is usually ca. 50 ppm; however, the efficiency depends on the When a vehicle tank is filled with diesel fuel, particular diesel base fuel. severe foam formation can occur because of Since dissolved metal ions (especially copper) intensive mixing with air. This has unpleasant can also catalyze the aging of diesel fuel, ca. effects: either the time necessary to fill the tank 5 ppm of a metal deactivator can be added. completely is extended or the amount of fuel Corrosion inhibitors (see Section 5.1.1), taken into the tank is considerably lower. which generally consist of high molecular mass Silicon-containing antifoams, mostly polysi- amines when used in diesel fuel, are also effecloxanes, have been developed and added to diesel tive against aging because of their basicity. fuel at a concentration of 10 – 50 ppm to avoid The detergents additives added to diesel fuel excessive foam formation. An effective antifoam principally to keep the injection nozzles clean are additive not only depresses foam formation but often effective in preventing diesel fuel aging. also accelerates foam decomposition. Additives for stabilizing diesel fuel are frequentExamples of suppliers are Goldschmidt (Te- ly used in the form of a multifunctional mixture at gopren series), OSI Witco (MR additives), and a concentration of ca. 100 ppm. Dow Corning. 5.2.7. Dehazers 5.2.6. Additives for Increasing Storage Stability – Antioxidants
During the storage of diesel fuel, aging and corrosion effects can occur in a way similar to gasoline (see Sections 5.1.3 and 5.1.4). The aging of diesel fuel also occurs via oxidation and radical polymerization reactions, which first cause darkening and later sediment formation [80]. The problem of unstable diesel fuel has increased particularly since ca. 1970 because higher proportions of diesel fuel components from cracking processes are in use [81]. These components have a higher content of unsaturated hydrocarbons
Through contact with water vapor in refinery processes and possible entry of water during transportation and storage, diesel fuel often becomes cloudy due to the presence of finely dispersed water droplets. Since such a cloudy, water-containing fuel would lead to considerable operational problems, the water droplets must have separated before the fuel is distributed. Especially in diesel fuels rich in aromatics, water separation may take a long time so that large tank capacities are necessary. The settling of water can be accelerated considerably by the addition of dehazers. These additives mainly lower the
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surface tension between water droplets, so that agglomeration to form larger, more rapidly sinking drops is achieved. Quaternary ammonium salts in a concentration of 5 – 50 ppm are effective dehazers. Examples of suppliers are Rechem (ER and EXP additives), Petrolite (Tolad additives), and Nalco (Nalco).
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small quantities of antistatic additives (up to 5 ppm) which, for example, contain calcium compounds is sometimes advisable where, very high pumping rates (e.g., at refineries, transfer, or truck refueling) can lead to the danger of static charge buildup. Electrical conductivity can be determined by means of a testing method ASTM D 2624, developed for jet fuel. Antistatic additives are supplied, e.g., by Octel (Stadis).
5.2.8. Biocides ( ! Biocides)
As explained in Section 5.2.7, the contact of diesel fuel with water cannot be ruled out totally. In some cases, water layers are formed at the bottom of the storage tank due to insufficient house keeping. At the interface between water and the diesel fuel, conditions are provided that allow the growth of microorganisms such as bacteria, yeast, and fungi. Infestation with microorganisms leads, on the one hand, to sediment formation which increases the danger of filter blocking. On the other hand, sulfate-reducing bacteria create a corrosive atmosphere that produces rust particles and can finally lead to tank destruction [82]. Through the addition of biocides, microbial infestation can be prevented or already affected tanks can be disinfected. Microorganisms can develop resistance to biocides under certain circumstances, so biocides should be used only sparingly for specific purposes. Generally careful cleaning of the affected storage tank and consequent removal of water from the tank solve the problem. A whole range of substances can act as biocides: formaldehyde derivatives, isothiazolones, triazines, N,N -methylenebis(5-methyloxazolidine), and boric acid derivatives. Biocides are usually applied in a range of 50 – 400 ppm. Biocides are supplied, e.g., by Sch€ u lke & Mayr, Norderstedt, Germany.
5.2.10. Reodorants
The odor of diesel fuel is often regarded to be unpleasant, with the result that people can reject diesel engines as ‘‘dirty’’. If the skin or clothes are contaminated with diesel fuel, the unpleasant odor can remain for a long time because of its low volatility. The unpleasant smell is caused particularly by certain sulfur-containing and unsaturated components. Desulfurization with hydrogen removes most of the unpleasant smelling ingredients. Modern low-sulfur fuels have generally a neutral smell so that specific reodorants are no longer required. Reodorants are based mostly on nature-identical substances that are toxicologically harmless, such as vanillin or terpenes. They usually work by blocking the olfactory nerves and can be added in concentrations of 30 – 100 ppm. Examples of suppliers are Haarmann & Reimer (Kompensols) and Bush Boake Allen (LG additives).
6. Fuel Standardization and Testing
The engine performance strongly depends on the physical and chemical properties of a fuel (see Chap. 3). To guarantee adequate fuel quality for a broad range of engine requirements, minimum specifications must be met, which are defined, for 5.2.9. Antistatic Additives example, in Europe in EN 228 (for gasoline) and When hydrocarbons are pumped rapidly, a build- EN 590 (for diesel) (Tables 9 and 10). up of static electricity can occur which, on disIn these specifications the great climatic difcharge, may lead to fuel vapor ignition. In com- ferences within Europe (Sweden – Portugal, for parison to kerosene, this danger is relatively example) are taken into account be defining small for diesel fuel because of its lower volatili- appropriate limits for, e.g., volatility of gasoline ty and its content of polar components that lead to and cold-flow performance of diesel fuels for increased conductivity. However, the use of each country.
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Table 9. European standard for gasoline; extract from EN 228 * **
r(15
3
C), kg/m Knock rating RON MON Lead content mg/L Distillation range, total proportion distilled, vol% up to 70 C:*** Winter and intermediate Summer up to 100 C: Winter and intermediate Summer Up to 150 C Final boiling point, C Vapor pressure, kPa *** Winter Intermediate and summer Benzene content, vol% Aromatics content, vol% Olefin content, vol% Sulfur content, mg/kg
Oxygenates, vol% Methanol Ethanol 2-Propanol (isopropyl alcohol, IPA) 2-Methyl-2-propanol (tert-butyl alcohol, IBA) 2-Methyl-1-propanol (isobutyl alcohol, JBA) Ethers containing 5 or more carbon atoms Other mono alcohols and ethers with a final boiling point below 210 C Oxygen content, mass% Induction period, min Copper corrosion visual rating
**
Regular
Super
Super Plus
Test method
720 – 775
720 – 775
720 – 775
EN ISO 3675, EN ISO 12185
min. 91.0 min. 82.5 max. 5
min. 95.0 min. 85.0 max. 5
min. 98.0 min. 88.0 max. 5
EN ISO 5164 EN ISO 5163 EN 237
22 – 50 20 – 48
22 – 50 20 – 48
22 – 50 20 – 48
EN ISO 3405
46 – 71 46 – 71 min. 75 210
46 – 71 46 – 71 min. 75 210
46 – 71 46 – 71 min. 75 210 EN 13016–1
60 – 90 45 – 60 1.0 max. 35.0 max. 21.0 max. 50 or max. 10
60 – 90 45 – 60 1.0 max. 35.0 max. 21.0 max. 50 or max. 10
60 – 90 45 – 60 1.0 max. 35.0 max. 21.0 max. 50 or max. 10
max. 3.0 max. 5.0 max. 10.0 max. 7.0 max. 10.0 max. 15.0 max. 10.0
max. 3.0 max. 5.0 max. 10.0 max. 7.0 max. 10.0 max. 15.0 max. 10.0
max. 3.0 max. 5.0 max. 10.0 max. 7.0 max. 10.0 max. 15.0 max. 10.0
max. 2.7 min. 360 max. 1
max. 2.7 min. 360 max. 1
max. 2.7 min. 360 max. 1
EN 238, EN 12177, EN 14517 ASTM D 1319, EN 14517 ASTM D 1319, EN 14517 EN ISO 20846, EN ISO 20847, EN ISO 20884, EN 1601, EN 13132
EN 1601, EN 13132 EN ISO 7536 EN ISO 2160
*
To reduce CO 2 emissions a minimum concentration of bio fuel components is legally required in Germany raising from 1.2% in 2007 stepwise by 0.8% per annum to 3.6% in 2010. ** Regular grade and Super plus are standardized locally, figures shown are valid for Germany. *** Mandatory limits vary depending on climatic conditions, figures shown are valid for central European climate.
The ASTM testing methods used in the United States are similar to the corresponding European (EN, ISO) methods. For all testing procedures a variation in measured values is unavoidable, and determination of a ‘‘true value’’ is impossible. The terms ‘‘true value’’, repeatability, and reproducibility are precisely defined in EN ISO 4259. The EN standards for gasoline and diesel fuel are mandatory. The quality of the fuels and their conformity with standards is, for example, tested on a random basis by the appropriate authorities in the Federal Republic of Germany and is followed up by court in the case of an offense. This is all the more important because not conforming to standards can cause damage to engines and
provide economic advantages for disreputable suppliers.
7. Storage and Transportation To transport fuels from the refinery to a vehicle tank a sophisticated infrastructure is necessary. Storage. In the refinery or in gasoline depots, fuels that are ready for use are first placed in cylindrical, generally steel tanks for intermediate storage. To store volatile gasoline fixed roof tanks with internal floating roofs are generally used. The roof, which is sealed against the wall of
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Table 10. European standard for diesel fuel; extract from EN 590 ****
Test method r (15
C), kg/m
3
Cetane number Cetane index Distillation range, total proportion recovered vol% up to 250 C up to 350 C 95% recovered Flash point C Viscosity (40 C), mm2 /s ** Filterability (CFPP), C Summer Winter Intermediate Sulfur content, mg/kg ***
820 – 845 min. 51 min. 46 Pr EN ISO 3405 below 65 min. 85 max. 360 C above 55 2.00 – 4.50 max. 0 max. 20 max. 10 max. 50 and max. 10
EN ISO 3675, EN ISO 12185 EN ISO 5165 EN ISO 4264
EN ISO 2719 EN ISO 3104 EN 116
max. 0.30
EN ISO 20846, EN ISO 20847, EN ISO 20884 EN ISO 10370
max. 0.01 max. 1
EN ISO 6245 ISO 2160
max. 25 max. 24 max. 200 max. 460
EN 12205 EN 12662 EN ISO 12937 EN ISO 12156–1 EN 12916
Conradson carbon residue, mass% Ash content, mass% Copper corrosion visual rating 3 Oxidation stability, g/m Sediment, mg/kg Water content, mg/kg Lubricity HFRR, m m* Polycyclic aromatics, mass%
max. 11
453
In the refineries or depots, vapor recovery plants with a retention efficiency of ca. 99% are sometimes employed for the filling of gasoline tankers [83]. When underground gasoline station tanks are being filled, displaced gasoline vapors are often returned to the tank trucks (vapor recovery stage 1). In Europe and the United States the emission of gasoline vapors is also controlled when car tanks are being filled by means of special dispensing nozzles (vapor recovery stage 2) [84]. Gasoline belongs to hazard class I, whereas diesel fuels are assigned to hazard class III because their flash point is > 55 C. Hazard classifications for the transport of gasoline and diesel fuel are as follows: Gasoline GGVS, ADR/RID IMDG Code UN no. ICAO/IATA-DGR
Class 3, number 3 b Class 3.1 1203 Class 3
Diesel fuel GGVS, ADR/RID IMDG Code UN no. ICAO/IATA-DGR
Class 3.3, number 31 c Class 3.3 1202 Class 3
*
Protection against wear of injection pumps defined in the High Frequency Reciprocating wear Rig (HFRR). ** Mandatory limits vary depending on climatic conditions, figures shown are valid for central European climate. *** The specification of 10 mg/kg supports the introduction of ‘‘sulfur free’’ Diesel fuels. **** To reduce CO 2 emissions a minimum concentration of 4.4% bio fuel components is legally required in Germany since 2007.
the tank, can rise and fall with the liquid level. To avoid hydrocarbon emissions the tanks breathe into a sealed common system (! Oil Refining, Section 4.2.1.1.). For the storage of diesel fuel, fixed roof tanks are frequently used. Small quantities of fuels can be stored in steel drums, steel cans or fuel-resistant plastic containers. Transportation. Fuels can be transported from the refineries via pipelines, tank ships, rail tankers, or road tank trucks. In some countries mineral oil companies exchange base fuels from their refineries. The shortening of the transport routes allows cost saving and reduces safety risks.
8. Alternative Fuels The search for alternative fuels is not new. During earlier efforts to introduce alternative fuels (e.g., during World War II or after the first oil crisis in 1973), concern about the extent of crude petroleum reserves or too great a dependence on oil-producing countries was predominant. Overcoming these problems was attempted by shifting fuel production from crude petroleum to other fossil energy sources. A true picture of the primary energy consumption can only be obtained by drawing up energy chains that take all conversion steps into account. Liquefied Petroleum Gas. (! Liquefied Petroleum Gas). With liquefied petroleum gas (LPG) an additional fraction of crude petroleum (predominantly propane – butane) can be used as fuel for Otto engines. Because of the relatively expensive modification costs for vehicles and other disadvantages (higher volumetric consumption, lower engine power) use of LPG has become established in only a few places where
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taxation is favorable (e.g., the Netherlands, Italy, Japan, Australia). Natural Gas. (! Natural Gas). Natural gas ( predominantly methane) can also be used as an engine fuel either as compressed natural (CNG) or as liquefied natural gas (LNG). However storage in liquefied form requires considerable sophistication. Until now, natural gas is only used as CNG on a broader scale in certain countries (Argentina, Italy, Russia). However, the relatively low carbon dioxide emission resulting from the high hydrogen content is favoring its use in the future. Various motor manufacturers have developed duel fuel vehicles for the use of CNG which are operated by fleet owners predominantly. Also a limited number of public service stations exist which offer CNG. Fuels from Fischer – Tropsch Synthesis. Synthetic gasoline or middle distillate can be produced from coal or natural gas via synthesis gas from the Fischer – Tropsch synthesis [85]. This method, however, is often associated with a very poor overall energy efficiency and has been used in only a few special cases (Germany: World War II; South Africa: Sasol Process). However, new catalysts and different process conditions allow nowadays an improved energy efficiency, e.g., the Shell Middle Distillate synthesis process which produces a high cetane quality diesel fuel component from natural gas on small scale. Methanol. Methanol is produced from synthesis gas [86], which can be obtained from crude oil, coal and even biomass. A number of experiments have been conducted on its use as a potential alternative fuel (Table 11). However, the use of almost pure methanol or of a 15% mixture with gasoline has not yet found a practical application. On the technical side, problems in cold- and hotrunning performance and aldehyde emissions with cold engine require special attention whereas the specific power output and engine efficiency is advantageous in comparison to gasoline engines. From the economic point of view the high consumption and high engine adaption costs are a disadvantage [87–89]. Ethanol. The use of ethanol, which can be obtained form crude oil, coal and biomass, as an
Vol. 4 Table 11. Important fuel properties for the performance of an Otto engine – comparison of methanol, ethanol, and regular gasoline [89]
Chemical composition, kg/kg fuel C H O Calorific value, MJ/kg RON MON Stoichiometric air – fuel ratio, kg air/kg fuel
Methanol (CH3OH)
Ethanol (C2H5OH)
Regular gasoline
0.38 0.12 0.50 19.7 ca. 114 ca. 94 6.4
0.52 0.13 0.35 26.8 ca. 111 ca. 94 9.0
0.86 0.14 43.0 ca. 92.6 ca. 82.7 14.7
alternative fuel has been put into practice in Brazil. Both neat ethanol (hydrated at about 95%) fuel and a 23% blend in gasoline are marketed. Ethanol is produced mostly from biomass and could therefore potentially help reduce carbon dioxide emissions because of a closed carbon dioxide cycle. On the other hand catalytic exhaust gas treatment is also required even when ethanol is used because of other emissions (CO, HC, NO x, and aldehydes). From the viewpoint of technical applications, the use of ethanol is somewhat less problematic than that of methanol [90, 91]; however, the high production costs and the huge cultivation area for biomass have kept its use as a fuel within narrow limits (see Table 12). Ethanol was used in Germany in former times (‘‘ Reichskraftstoff ’’: 1/3 ethanol, 1/3 benzene, 1/3 gasoline). However since 2007 ethanol produced from biomass, either directly but more so converted to ETBE (ethyl-tertiary-butyl-ether) is used again in low concentrations as blending component in gasoline to meet the legally required minimum of biomass components installed to reduce CO2 emissions (see Chap. 6). Fatty Acid Methyl Esters (FAME) are produced from vegetable oils by esterification. They are regarded as ‘‘first-generation bio fuels’’. Especially neat rape seed oil methyl ester (RME) is used as a fuel for diesel engines on a small scale in some countries (mainly France, also Austria [92] and Germany). However since 2007 it is used as blending component in conventional diesel fuel to meet the legal requirement of 4.4% minimum concentration of bio fuel components in diesel fuel to reduce CO2 emissions. In principle vegetable oil fuels offer the advantages
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Table 12. Fuels from biomass
Raw material
Average yield of harvest, t ha1 a1
Wood Sugar beet Potatoes Wheat Maize Rape seed
10.0 46.0 25.0 4.5 5.0 2.5
a
Crude ethanol/ crude rape seed oil, L ha1 a1
Acreage per volume unit of fuel, m 2 /L
Energy per acreage unit kW h m2 a1
Net energy profit [93] including production expense, %
1.500b 4.600b b 2.850 1.760b b 2.000 c 1.250
6.7 2.2 3.5 5.7 5.0 8.0
0.88 2.71 1.68 1.04 1.18 1.26
0 20 20 10 20 35
a
The amount harvested is related to energy farming. Crude ethanol. c Crude rape seed oil. b
Table 13. Properties of various vegetable oils in comparison with diesel fuel
Property
Density Viscosity (40 C), mm2 /s Sulfur, ppm Flash point, C Cetane number Lower calorific value, MJ/kg Appearance Low-temperature behavior (limiting temperature for operability), C *
Fuels Sunflower seed oil
Soybean oil
Rape seed oil
Diesel fuel
0.924
0.923
0.920
0.830
31.0 50 196 33
31.2 50 220 27
34.4 70 202 40
2.8 1500 60 53
36.2 slightly cloudy, yellow > 0
39.4 clear, dark amber-colored > 0
35.2 clear, amber-colored þ 15
42.6 clear, bright 22
a
Typical values for Germany.
of greenhouse gas emissions benefits because of the closed carbon cycle, new outlets for agricultural products, the possibility of usage in standard diesel engine with minimum modification, and good biodegradability. On the other hand they offer little improvement over conventional diesel fuels in view of local air quality. Although exhaust smoke is somewhat lower, regulated and nonregulated emissions are scarcely different form those of diesel fuel. The global impact of the potential reduction in carbon dioxide emissions is limited by production capacity of RME which is predicted to rise but will still be less than 10% of the diesel market [93, 94]. Vehicles fueled with RME show reduced power due to the lower calorific value and the higher viscosity as compared with diesel fuel. The volumetric fuel consumption is higher because of the oxygen content of RME. Technically, the use of rape seed oil fuel is also associated particularly with an increased engine wear under
certain circumstances (caused by deposit formation) and poor cold flow performance (Table 13). Hydrogen. can be used either as an engine fuel or to feed fuel cells for on board generation of electricity. Neither of these concepts has gained a commercial status so far, partly because of the difficulties connected with the handling and storage of hydrogen (see Tables 14 and 15). Table 14. Energy storage in motor vehicles — energy equivalent to 50 L of gasoline
Gasoline Diesel fuel Methanol Ethanol Hydrogen (liquid) Metal hydride (FeTi) Electricity, NaS battery
Total tank volume, L
Tank mass, kg
Tank filling time, min
55 50 103 84 330 385 3565
47 47 88 72 94 1175 3800
2 2 2 2 7 – 80 20 600
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Table 15. Environmental compatibility and availability of various fuels
Fuel
Environmentally sensitive substances
Availability
Gasoline (with catalyst) Diesel fuel LPG Methanol fossil
CO2, ozone among others
worldwide
CO2, particle, NO x, ozone CO2, among others
worldwide some countries
CO2, formaldehyde
biomass Ethanol (biomass) Vegetable oils
formaldehyde acetaldehyde particles, among others
locally possible in short time ? Brazil possible locally to some extent
H2 fossil nonfossil Electricity fossil nonfossil
CO2, NO x, among others
? ?
CO2, NO x, among others
worldwide possible to some extent
For detailed information see ! Hydrogen, 6. Uses, Section 2.3.3.3.. Electricity. For the use of electricity as an alternative fuel, global environmental compati-
bility is again determined by the production method. Electricity production based on fossil energy sources also leads to carbon dioxide emission as well as to the formation of regulated harmful substances. In addition, the total energy efficiency (based on the energy chain from production to utilization) of electrically driven vehicles is clearly lower than that of conventional fuels. However, the use of electric vehicles for areas of higher emission sensitivity (indoor areas, city centers) could become increasingly important. The storage of electricity in the vehicle represents a great problem because of the weight and the size of the batteries [95]. The development of efficient batteries such as the sodium – sulfur battery led to an increase in the energy density by a factor of ca. 4 in comparison with the lead – acid battery. However, compared to liquid fuels currently in use, there is still a considerable disadvantage in energy storage (and therefore in vehicle range). A solution to this problem is the onboard production of electricity by fuel cells which are currently under development (! Fuel Cells, Section 4.1. Zero Emission Vehicle). Ultimately these can be fueled with hydrogen. Because of the handling and availabil-
Table 16. Evaluation of alternative systems
Diesel LPG Gasoline
a
Natural Hydrogen b gas
b
Bioethanol
b
Methanol
b
Fuel cell/ hydrocarbon Fuel
Electricity
Without With catalytic catalytic converter converter Suitability Availability Economy Infrastructure
CO HC NO x Particulates CO2 Nonregulated emissions
0 0 0 0
0 0 0 0 0 0
0 0 0 0
þþ þþ þþ þ 0
þþ
0 0
þ 0
þþ þþ þ þ þþ
þ /
þ þ þþ þ 0
þþ
0
þ þ þþ þþ þþ þþ
þþþ þþþ þ þþþ þþþ þþþ
0/
þ þe þ þ þþ
d
/0
þ þ þ þ
e
0/
0 0
þ 0 from fossil fuels
þþþ þþþ þþþ þþþ þ þþþ
c
from nuclear and solar technology
þ þ
þþþ þþþ þþþ 0 þþþ þþþ 0
radioactivity from nuclear production?
0 similar; þ somewhat better; þþ clearly better; þþþ very muchbetter; somewhat worse; clearly worse; verymuch worse. a Dual system without catalyst. b With catalyst. c For electricity: 0. d Except Brazil. e Aldehydes: .
Vol. 4
ity problems use of intermediate fuels such as hydrocarbons and methanol are currently discussed. The fuel cell technology has also the advantage of a noticeably improved efficiency compared to the combustion engine. Hybrid Concepts. The introduction of alternative fuels is associated with considerable changes in the infrastructure that cannot be carried out in short term. For this reason, dualfuel hybrid concepts can greatly ease the introduction of new fuels. Up to now, prototypes for the alternative use of gasoline – methanol (‘‘flexifuel’’ concept) [87], gasoline – hydrogen [96], and diesel – electricity [97], [97] have been developed. Gasoline – LPG and gasoline – CNG vehicles are commercially available in some areas, however, their market share is small mainly because of the additional initial costs and the reduced load capacity because of the second tank. At present, considerable discrepancy still exists between environmental compatibility and other important criteria such as suitability, availability, economic factors, and infrastructure (Table 16) [98].
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